Memory comes in three popular forms: RAM chips, ROM chips, and disks.

You already learned about RAM chips and ROM chips. Letís examine disks.

A computer disk is round, like a phonograph record.

Three kinds

You can buy three popular kinds of computer disks:

A floppy disk is made of flimsy material. Itís permanently encased in a sturdy, square dust jacket.

A hard disk is made of firmer material. It typically hides in your computer permanently, unseen.

A CD-ROM is a compact disk. Itís the same kind of CD compact disk that plays music.

Each kind has its own advantages and disadvantages.

Floppy disks are the cheapest (under 50Ę per disk) and the easiest to mail to your friends: just stick the floppy disk in an envelope, perhaps with some padding. Unfortunately, floppy disks work the most slowly, and they hold the least data: the typical floppy disk holds about 1 megabyte, while the typical hard disk or CD-ROM can hold many hundreds of megabytes.

Hard disks work the fastest ó over 20 times faster than the other kinds! But hard disks are also the most expensive. Moreover, they typically canít be removed from your computer and therefore canít be mailed to your friends.

CD-ROMs are the best value: CD-ROM disks cost less than 1Ę per megabyte to manufacture. But they have a frustrating limitation: the information on CD-ROM disks cannot be edited.

Since each kind of disk has its own advantages and disadvantages, youíll want to buy all three kinds.


Computer experts argue about spelling. Some experts write "disk", others write "disc".

Most manufacturers write "disk" when referring to floppy disks or hard disks, but write "disc" when referring to CD-ROMs. That inconsistency annoys me.

To be more consistent, Iíll always write "disk", even when referring to CD-ROMs. Most computer magazines (such as PC Magazine and PC World) feel the same way I do: they always write "disk". The growing tendency is to always write "disk".

For hard disks, IBM used to write "disc" but now writes "disk".

Floppy disks

A floppy disk (or diskette) is round but comes permanently sealed in a square dust jacket. (Donít try to remove the floppy disk from its square jacket.)

The floppy disk is as thin and flimsy as a sheet of paper but is protected by the sturdy, square jacket that encases it.

Three standard sizes

Floppy disks come in three standard sizes.

The most popular size is called a 3Ĺ-inch floppy disk, because it comes in a square jacket thatís about 3Ĺ inches on each side. (Actually, each side of the jacket is slightly more than 3Ĺ inches, and the diskís diameter is slightly less.)

An older size, used mainly on older computers, is called 5ľ-inch. It comes in square jacket thatís exactly 5ľ inches on each side.

An even older size, 8-inch, is used just on ancient computers that are no longer built.

Those three sizes have nicknames:

An 8-inch floppy disk is called a large floppy.

A 5ľ-inch floppy disk is called a minifloppy.

A 3Ĺ-inch floppy disk is called a microfloppy.

Jacket colors

The jacket of a 5ľ-inch or 8-inch floppy disk is usually black. The jacket of a 3Ĺ-inch floppy disk is usually black, blue, white, or beige (very light grayish brown).

If you pay a surcharge, you can get jackets that have wilder colors.


8-inch floppies were invented in the early 1970ís by IBM. 5ľ-inch floppies were invented in the late 1970ís by Shugart Associates, which later became part of Xerox.

3Ĺ-inch floppies were invented in the 1980ís by Sony. Theyíve become the most popular size because theyíre the smallest, cutest, and sturdiest. Theyíre small enough to fit in the pocket of your shirt, cute enough to impress your friends, and sturdy enough to survive when you fall on your face. Theyíre also easy to mail, since theyíre small enough to fit in a standard white business envelope and sturdy enough to survive the U.S. Postal System. Yup, nice things come in small packages!

Magnetized iron

The round disk (which hides inside the square jacket) is coated with rust, so it looks brown. Since the rust is made of iron, which can be magnetized, the disk stores magnetic signals. The pattern of magnetic signals is a code representing your data.


To use a floppy disk, you must buy a floppy-disk drive, which is a computerized record player.

If the drive is external, itís a box sitting near the computer. If the drive is internal, itís built into the middle of the computer.

The drive has a slit in its front side. To use the drive, push the disk (including its jacket) into the slit.

When pushing the sheathed treasure into the boxís slit, donít shove too hard. Oooh! Please be gentle!

When you push your disk into the slit, donít push the disk in backwards or upside-down! Hereís how to push the disk in correctly:

First, notice that the diskís jacket has a label on it and also has a big oval cutout. (If the disk is 3Ĺ-inch, the cutout is covered by a metal slider.) Insert the disk so that the oval cutout goes into the drive before the label does. If the driveís slit is horizontal, make sure the label is on the top side of the jacket; if the drive is vertical, make sure the label is on the left side of the jacket.

After putting the disk into the slit, close the latch to cover the slit. (If the disk is 3Ĺ-inch, there is no latch.) Since the slit and latch act as a door, closing the latch is called closing the door.

As soon as you close the door, the disk drive automatically positions the disk onto the turntable thatís hidden inside the drive. The turntableís called the spindle. It can spin the disk quickly.

Like a record player, the disk drive contains an arm with a "needle" on it. The needle is called the read-write head, because it can read whatís on the disk and also write new information onto the disk.

Hereís how to write new information onto the disk. Put your fingers on the computerís keyboard. Type a command that tells the computer you want to use the disk. Then type the information you want to transfer to the disk.

To transfer the information to the disk, the computer lowers the read-write head onto the disk. An electrical charge passes through the head. The charge creates an electromagnetic field, which magnetizes the iron on the diskís surface. Each iron particle has its own north and south pole; the patterns formed by the north and south poles are a code that stands for the information youíre storing.

Tracks As the disk spins, the head remains stationary, so that the head draws a circle on the spinning diskís surface. The circleís called a track. To draw the circle, the head doesnít use ink; instead, it uses a pattern of magnetic pulses. Since your eye canít see magnetism, your eye canít see the circle; but itís there!

When you start using a blank disk, the arm puts the head near the diskís outer rim, so that the headís track (circle) is almost as wide as the disk. That trackís called track 0.

Then the arm lifts the head, moves the head slightly closer to the virgin diskís center, and puts the head back down onto the disk again. The head draws another circular track on the disk, but this new circular track is slightly smaller than the previous one. Itís called track 1.

Then the head draws track 2, then track 3, then track 4, and so on, until the head gets near the center of the disk, and draws the last circular track (which is smaller than the other tracks).

To organize the information on a track, the computer divides the track into sectors. Each "sector" is an arc of the circle.

Single-sided versus double-sided drives A modern disk drive has two read-write heads. One head uses the diskís top surface, while the other head uses the diskís bottom, so that the drive can use both sides of the disk simultaneously. Thatís called a double-sided disk drive. The drive puts information onto the disk by first using track 0 of the main side, then track 0 of the flip side, then track 1 of the main side, then track 1 of the flip side, etc.

If a disk drive is not modern ó if itís ancient and primitive ó it has just one read-write head, which uses just one side of the disk. The flip side of the disk is unused. That kind of drive is called a single-sided disk drive. Which side of the disk does the drive use? Though some drives use the side that has the label, other drives (by other manufacturers) use the side opposite the label instead.

Double-sided is also called DS and 2-sided and 2S. Single-sided is also called SS and 1-sided and 1S.

Capacity How many kilobytes can you fit on a floppy disk? The answer depends on which kind of drive you have.

The most popular kind of drive is called a 3Ĺ-inch high-density floppy drive. Hereís how it works.

It holds a 3Ĺ-inch floppy disk. It writes on both sides of the disk simultaneously, since itís a double-sided disk drive. It writes 80 tracks on each side. It divides each track into 18 sectors. Each sector holds "512 bytes", which is half a kilobyte, ĹK.

Since the disk has 2 sides, 80 tracks per side, 18 sectors per track, and ĹK per sector, the diskís total capacity is "2 times 80 times 18 times ĹK", which is 1440K. So altogether, the disk holds 1440K. Thatís called 1.44M (where an M is defined as being 1000K). Thatís why a 3Ĺ-inch high-density floppy drive is also called a 1.44M drive.

The kind of disk you put into it is called a 1.44M floppy disk (or a 3Ĺ-inch high-density floppy disk). Since the disk holds 1.44M (which is 1440K), and since a K is 1024 bytes, the disk holds "1440 times 1024" bytes, which is 1,474,560 bytes altogether. Thatís a lot of bytes!

Although the disk holds 1440K, some of those K are used for "bureaucratic overhead" (such as holding a directory that reminds the computer which data is where on your disk). A Mac uses just 1 sector (ĹK) for bureaucratic overhead. An IBM-compatible computer uses 33 sectors (16ĹK) for bureaucratic overhead, leaving just 1423ĹK (1,457,664 bytes) for your data.

When you buy a blank disk to put in a 1.44M drive, make sure the disk is the right kind. Make sure the disk is 3Ĺ-inch; and to get full use of what the drive can accomplish, make sure the disk is high-density! The abbreviation for "high-density" is HD. A high-density 3Ĺ-inch disk has the letters HD stamped in white on its jacket; but the H overlaps the D, so it looks like this: HD. Also, a high-density 3Ĺ-inch disk has an extra square hole cut through its jacket.

Old computers use inferior floppy drives, whose capacities are below 1.44M.

A capacity below 150K is called single-density (SD).

A capacity above 150M but below 1M is called double-density (DD).

A capacity above 1M is called high-density (HD).

Anything below high-density is called low-density.

Although the jacket of a high-density 3Ĺ-inch disk has "HD" stamped on it and an extra hole punched through it, the jackets of other kinds of disks often lack any distinguishing marks. Too bad!

Popular IBM-compatible drives For IBM-compatible computers, four kinds of floppy drives have been popular:

IBM driveís name Capacity Details

5ľ-inch double-density 360K 40 tracks per side, 9 sectors per track

5ľ-inch high-density 1200K (which is 1.2M) 80 tracks per side, 15 sectors per track

3Ĺ-inch double-density 720K 80 tracks per side, 9 sectors per track

3Ĺ-inch high-density 1440K (which is 1.44M) 80 tracks per side, 18 sectors per track

Each of those IBM-compatible drives is double-sided and has ĹK per sector. Theyíre manufactured by companies such as NEC, Teac, Chinon, Epson, and Alps. The fanciest drives (3Ĺ-inch high-density) used to be expensive, but now you can buy them for just $29 from mail-order discount dealers (such as USA Flex at 444 Scott Dr., Bloomingdale IL 60108, phone 800-723-2261 or 708-582-6206).

Mac drives For Mac computers, three kinds of floppy drives have been popular:

Mac driveís name Capacity Details

1-sided double-density 400K 1 side, 8-12 sectors per track

2-sided double-density 800K 2 sides, 8-12 sectors per track

high-density 1440K (which is 1.44M) 2 sides, 18 sectors per track

Each Mac drive is 3Ĺ-inch and has 80 tracks per side, ĹK per sector. The Macís high-density drive is called the Mac Superdrive.

On a disk, the inner tracks have smaller diameters than the outer tracks. Most drives squeeze as many sectors onto an inner track as onto an outer track, but the Mac double-density drives puts fewer sectors onto the inner tracks and put extra sectors onto the outer tracks. Specifically, the outer 16 tracks are divided into 12 sectors, the next 16 tracks into 11 sectors, the next 16 into 10, the next 16 into 9, and the inner 16 into 8.

Drives for other computers For other computers, many kinds of floppy drives have been invented:

Computer Drive capacity Details

Apple 2 family 140K 5ľ", 1 side, 35 tracks, 16 sectors, ľK per sector

Tandy Color Computer 157ĹK 5ľ", 1 side, 35 tracks, 18 sectors, ľK per sector

Tandy Models 3, 4, 4P 180K 5ľ", 1 side, 40 tracks, 18 sectors, ľK per sector

Tandy Model 4D 360K 5ľ", 2 sides, 40 tracks, 18 sectors, ľK per sector

Commodore 64 170ĺK 5ľ", 1 side, 35 tracks, 17-21 sectors, ľK per sector

Commodore Amiga 880K 3Ĺ", 2 sides, 80 tracks, 11 sectors, ĹK per sector

For the Commodore 64, the 17 outer tracks are divided into 21 sectors, the next 7 tracks into 19 sectors, the next 6 tracks into 18 sectors, and the inner 5 tracks into 17 sectors.

Speed In the disk drive, the disk spins quickly. The exact speed depends on what size disk the drive uses.

Low-density 5ľ-inch disks revolve 5 times per second. That makes 300 revolutions per minute, 300 rpm.

8-inch disks and high-density 5ľ-inch disks revolve faster: 6 times per second (360 rpm).

3Ĺ-inch disks revolve even faster: between 6Ĺ and 10 times per second.

Buying disks

When you buy a floppy disk, make sure its size matches the size of the drive. For example, a 3Ĺ-inch disk will not work in a 5ľ-inch drive.

If you buy a blank 5ľ-inch floppy disk, you can stick it into any normal 5ľ-inch drive, regardless of who manufactured the drive and who manufactured the computer. But after youíve put information onto the disk, that information is understandable only to your kind of computer. For example, an Apple 2e cannot understand what an IBM PC writes.

When you go into a computer store to buy a disk that contains software, tell the salesperson which kind of computer you have, so that the salesperson can give you a disk containing information understandable to your computer.

If your drive is single-density or double-density, it cannot handle high-density disks at all.

If your drive is 5ľ-inch and high-density, it can read single-density and double-density disks, but it might have trouble writing new information onto them. So when buying blank disks for your 5ľ-inch high-density drive to write on, avoid buying single-density or double-density disks.

The three crummy kinds of 5ľ-inch floppy disks (single-sided single-density, single-sided double-density, and double-sided double-density) are all manufactured by the same process as each other. The only difference is the manufacturerís "guarantee": a double-sided double-density disk is "guaranteed" to work on both sides and hold lots of data; a single-sided or single-density disk is not. Even if you buy a disk that has a poor guarantee (just "single-sided single-density"), it typically works fine even if you use both sides and store lots of data. The only difference is that the manufacturer hasnít bothered testing the second side and hasnít bothered testing double-density data. During the 1970ís and 1980ís, single-sided single-density disks were significantly cheaper than double-sided double-density, but now the prices are about the same.

Formatting the disk Before you can use a blank floppy disk, its surface must be formatted (divided into tracks and sectors). Buy a disk thatís been formatted already, or buy an unformatted disk and format it by typing a command on your computerís keyboard.

After the diskís been formatted, you can store whatever information you wish onto the disk. Do not tell the drive to format that disk again. If you accidentally make the drive format the same disk again, the drive will create new tracks and sectors on the disk, and erase the old tracks and sectors, and therefore erase all your old data!


If a disk is blank, format it before you use it.

If a disk already contains info, do not format it; itís been formatted already.

Name brands The most famous manufacturers of floppy disks are Verbatim and Maxell. But instead of buying those brands, buy generic floppy disks instead. The generics cost less and typically work just as well.

Discount dealers To get the lowest prices on generic floppy disks, contact MEI Micro Center (1100 Steelwood Rd., Columbus OH 43212, 800-634-3478) or Diskettes Unlimited (6206 Long Dr., Houston TX 77087, 800-DOG-DISK).

For example, here are the prices from MEI Micro Center for double-sided disks:

Kind of disk 100 disks 1000 disks

5ľ-inch double-density, unformatted $19+$1.20 $160+$12

5ľ-inch high-density, unformatted $26+$1.20 $210+$12

3Ĺ-inch high-density, formatted $28+$2.40 $250+$24

Add up the prices of what you want, then add the handling charge ($3.25). For example, for 100 of the best disks (3Ĺ-inch high-density, formatted), MEI charges you $28 (for the disks) + $2.40 (shipping) + $3.25 (handling), which is $33.65. Thatís about 34Ę per disk. For 1000 of the best disks, MEI charges you $250 + $24 + $3.25, which is $277.25, which comes to about 28Ę per disk. Diskettes Unlimited charges even less but might give you slightly lower quality; for details, phone them.

Whatís a disk worth? Although you can buy a blank floppy disk for under 50Ę, a disk containing information costs much more. The price depends on how valuable the information is. A disk that explains to the computer how to play a game costs about $25. A disk teaching the computer how to handle a general business task (such as accounting, filing, or correspondence) usually costs about $100.

A disk containing intimate, personal data about your businessís customers, suppliers, employees, and methods is worth even more ó perhaps thousands of dollars! To compute how much itís worth to you, imagine that youíve lost it, or that it fell into the wrong hands!

Protect your disks

Most parts of a computer system are sturdy: even if you bang on the keyboard and rap your fist against the screen, you probably wonít do any harm. Only one part of a computer system is delicate: that part is the disk. Unfortunately, the magnetic signals on your disk are easy to destroy.

One way to accidentally destroy them is to put your disk near a magnet; so keep your disks away from magnets! For example, keep your disk away from paper clips that have been in a magnetized paper-clip holder. Keep your disk away from speakers (such as the speakers in your stereo, TV, and phone), because all speakers contain magnets. Keep your disk away from electric motors, because motors generate an electromagnetic field. So to be safe, keep your disk at least six inches away from paper clips, stereos, TVís, telephones, and motors.

Keep your disk away from heat, because heat destroys the diskís magnetism and "melts" your data. So donít leave your disk in the hot sun; donít leave it on a sunny windowsill; donít leave it in the back of your car on a hot day. If your disk drive or computer feels hot, quickly lower the temperature, by getting an air conditioner or at least a fan.

3Ĺ-inch floppy disks come in strong jackets, but 5ľ-inch and 8-inch floppy disks come in jackets that are too weak and thin to protect disks from pressure. Donít squeeze your disk. Donít put it under a heavy object, such as a paperweight or a book. If you want to write a note on the diskís jacket, donít use a ball-point pen (which crushes the disk); use a soft felt-tip pen instead.

Keep the disk away from dust. For example, donít smoke cigarettes near the disk, because the smoke becomes dust that lands on the disk and wrecks the data.

Keep the disk dry. If you must transport a disk during a rainstorm, put the disk in a plastic bag. Never drink coffee or soda near the disk: your drink might spill.

To handle the disk, touch just the diskís jacket, not the brown disk itself. Holes in the jacket let you see the brown disk inside; donít put your fingers in the holes.

Power surges in ancient computers If your computerís an IBM clone or by Apple, skip ahead to the next topic ("Write-protect notch").

If your computer is made by Commodore or Radio Shack and is so ancient that itís not an IBM PC clone, be careful: flipping the power switch on your ancient computer creates an electrical surge that wrecks the disk. On such a computer, donít flip the power switch when the drive contains a disk. Flip the power switch just when the driveís empty.

To turn such a computer on, make sure the driveís empty, then flip the power switch on. After the powerís come on, insert the disk.

Before turning such a computer off, remove the disk from the drive. When the driveís empty, turn off the power.

Write-protect notch When you buy a blank 5ľ-inch or 8-inch floppy disk, the disk comes in a square black jacket. Since the jacketís square, it has four sides; but one of the sides has a notch cut into it.

You can cover the notch, by sticking a plastic tab over it. The tab has a gummed back, so you can stick it on the disk easily and cover the notch. You get the tab free when you buy the disk.

(For a 3Ĺ-inch disk, the notch is different: itís a square hole near the jacketís corner but not on the jacketís edge. To cover it, you use a black slider instead of a tab. On old Apple Mac disks, the slider was red instead of black.)

Whenever you ask the computer to change the info on the disk, the drive checks whether youíve covered the notch.

For a 5ľ-inch disk, the normal situation is for the notch to be uncovered. For a 3Ĺ-inch or 8-inch disk, the normal situation is for the notch to be covered.

If the situationís normal, the computer will obey your command: it will change the info on the disk as you wish. But if the situationís abnormal (because the notch is covered when it should be uncovered, or is uncovered when it should be covered), the computer will refuse to change the diskís info.

Suppose your disk contains valuable info, and youíre afraid some idiot will accidentally erase or alter that info. To prevent such an accident, make the situation abnormal (by changing whether the notch is covered), so that the computer will refuse to change the diskís info. It will refuse to erase the disk; it will refuse to add new info to the disk; it will refuse to alter the disk; it will refuse to write onto the disk. The disk is protected from being changed; itís protected from being written on. The disk is write-protected (or locked).

Since the tab affects whether the disk is write-protected, the tab is called a write-protect tab, and the notch is called a write-protect notch.

When you buy a disk that already contains info, the disk usually comes write-protected, to protect you from accidentally erasing the info. So if you buy a 5ľ-inch floppy disk that already contains info, it might come with a write-protect tab already covering the notch, to write-protect the disk.

Instead of creating a notch and then covering it with a tab, some manufacturers save money by getting special disks that have no notch. The computer treats a notchless disk the same way as a disk whose notch is covered.

Backup Even if you handle your disk very carefully, eventually something will go wrong, and some of the info on your disk will get wrecked accidentally.

To prepare for that inevitable calamity, tell the computer to copy all info from the disk onto a blank disk, so that the blank disk becomes an exact copy of the original. Store the copy far away from the original: store it in another room, or ó better yet ó another building, or ó better yet ó another city.

The copy is called a backup. Use the backup disk when the original disk gets wrecked.

Making a backup disk is like buying an insurance policy: it protects you against disasters.

When you buy a floppy that already contains software, try copying the floppy before you begin using it.

If youíre lucky, the computer will make the backup copy without any hassles. If youíre unlucky, the software company has put instructions on the floppy that make the computer refuse to copy the disk, because the company fears that youíll illegally give copies to all your friends for free. A floppy that the computer refuses to copy, and which is therefore protected against illegal copying, is called copy-protected. A floppy that you can copy is called copyable (or unprotected).

Drive cleaners Donít bother trying to clean the heads of your floppy drive. The heads donít collect much dirt anyway, since the floppy diskís jacket has a cloth liner that traps most dirt. If your disk ever starts to act unreliable, clean the heads if you wish, but the culprit is more likely a misaligned head, a brownout, overheating, defective software, or a mistyped command.

Super-capacity floppies

A standard floppy disk holds up to 1.44M. Super-capacity floppy disks can hold even more.

Zip The most popular super-capacity floppy disk is the Zip disk. It looks the same size as a standard floppy disk (3Ĺ inches) but holds 100M, which is 70 times as much as a 1.44M floppy! It costs $18, but you can pay just $11 each by getting 6 at a time.

To use Zip disks, you must buy a Zip drive. It reads just Zip disks and costs $139. (That price includes one blank Zip disk.)

The Zip drive is made by Iomega. Zip disks are available from many manufacturers, such as Iomega and Sony. You can buy Zip drives and disks at your local computer store.

LS-120 The next step up is the laser servo 120M disk (LS-120 disk). Like a 1.44M disk and a Zip disk, an LS-120 disk is 3Ĺ-inch, but it holds more (120M). It costs $18, but you can pay just $16 each by getting 5 at a time.

To use LS-120 disks, you must buy an LS-120 drive. It reads LS-120 disks and 1.44M disks. It costs $170. Itís faster than a Zip drive. You can buy LS-120 drives made by O.R. Technology and Imation.

PD The final step up is the Panasonic disk (PD). Itís 5-inch and holds 650M. It costs $34, but you can pay just $30 each by getting 10 at a time.

To use PD disks, you must buy a PD drive. It reads PD disks and CD-ROM disks. It costs $349. You can buy PD drives made by Panasonic, Toray, and Micro Solutions.

Hard disks

Hard disks are better than floppy disks in three ways:

Hard disks are sturdier than floppies.

Hard disks are hard and firm; they donít flop or jiggle.

Theyíre more reliable than floppies.

Hard drives hold more information than floppy drives.

The typical floppy drive holds 1.44 megabytes.

The typical hard drive holds 4 gigabytes (which is about 4000 megabytes).

Hard drives work faster than floppies.

The typical floppy disk rotates between 5 and 10 times per second.

The typical hard disk rotates between 60 and 120 times per second.

Hard drives cost more than floppy drives. The typical floppy drive costs $29; the typical hard drive costs about $240.

Unfortunately, the typical hard disk canít be removed from its drive: the hard disk is non-removable, stuck inside its drive permanently. (Hard disks that are removable are rare.)

Since the typical hard disk is stuck forever inside its drive, in one fixed place, itís called a fixed disk.

Though the typical floppy-disk drive holds just one disk at a time, the typical hard-disk drive holds a whole stack of disks and handles all the stackís disks simultaneously, by using many arms and read-write heads.

For example, a 4-gigabyte hard drive holds a non-removable stack of disks, and the entire stack totals 4 gigabytes. Each disk in the stack is called a platter. If your hard drive is the rare kind that holds a removable stack of disks, the stack comes in a cartridge or pack that you can remove from the hard drive.

Back in 1977, the typical hard disk had a 14-inch diameter and was removable. The hard-disk drive was a big cabinet, the size of a top-loading washing machine; it cost about $30,000 and held 100 megabytes. It required a minicomputer or mainframe.

Hard disks, drives, and prices have all shrunk since then! Now the typical hard disk has a diameter of just 3Ĺ inches. The typical hard drive is just 1 inch tall, costs about $240, and holds 4300 megabytes. It fits in a desktop microcomputer.

Some notebook computers use tiny hard disks whose diameter is just 2Ĺ inches.

IBM drive letters

The typical IBM-compatible computer has both a floppy drive and a hard drive. The floppy drive is called drive A; the hard drive is called drive C.

If the computer has two floppy drives,

the main floppy drive is called drive A; the other floppy drive is called drive B.

If the computer has two hard drives,

the main hard drive is called drive C; the other hard drive is called drive D.

Copy from floppy to hard & back

When you buy a program, it usually comes on a floppy disk. To use the program, put that floppy disk into the floppy drive, then copy the program from the floppy disk to the hard disk. (To copy the program onto an IBM-compatible hard disk, type the word "copy" or "install" or "setup". To find out which of those three words to type and when, follow the instructions in the manual that came with the program.)

Then use just the copy on the hard disk (which is sturdier, holds more info, and works faster than the floppy disk).

Like floppy disks, hard disks are coated with magnetized iron. Floppy disks and hard disks are both called magnetic disks. Like floppy disks, hard disks are in constant danger of losing their magnetic signals ó and your data!

Protect yourself! Every day, take any new info thatís on your hard disk and copy it onto a pile of floppy disks, so those floppy disks contain a backup copy of what was new on your hard disk.

To avoid giant disasters, avoid creating giant files. If youíre writing a book and want to store it on your hard disk, split the book into chapters, and make each chapter a separate file, so if you accidentally say "delete" youíll lose just one chapter instead of your entire masterpiece.

How the head works

In a floppy drive, the read-write head (the "needle") touches the spinning floppy disk. But in a hard drive, the read-write head does not touch the spinning hard disk; instead, it hovers over the disk.

The distance from the read-write head to the hard disk is a tiny fraction of an inch, and small enough so that the read-write head can detect the diskís magnetism and alter it.

Since the head doesnít actually touch the disk, there isnít any friction, so the head and the disk donít suffer from any wear-and-tear. A hard-disk system therefore lasts longer than a floppy-disk system and is more reliable.

Winchester drives In all modern hard drives, the head acts as a miniature airplane: it flies above the disk. It flies at a very low altitude: a tiny fraction of an inch. The only thing keeping the head off the rotating disk is a tiny cushion of air ó a breeze caused by the diskís motion.

When you unplug the drive, the disk stops rotating, so the breeze stops, and the head comes to rest on a landing strip, which is like a miniature airport.

Such a drive is called a flying-head drive. Itís also called a Winchester drive, because "Winchester" was IBMís secret code-name for that technology when IBM was inventing it.

The head flies at an altitude thatís extremely low ó about a ten-thousandth of an inch! Thatís even smaller than the width of a particle of dust or cigarette smoke! So if any dust or smoke lands onto the disk, the head will smash against it, and youíll have a major disaster.

To prevent such a disaster, the entire Winchester drive is sealed air-tight, to prevent any dust or smoke from entering the drive and getting onto the disk. Since the drive is sealed, you canít remove the disks (unless you buy an extremely expensive Winchester drive that has a flexible seal).


Hereís how the computer retrieves data from the drive.

First, the driveís head moves to the correct track.

The time that the head spends moving is called the seek time. Since that time depends on how far the head is from the correct track, it depends on where the correct track is and where the head is moving from.

According to calculus, on the average the head must move across a third of the tracks to reach the correct track. Thatís why the time to traverse a third of the tracks is called the average seek time.

A millisecond (ms) is a thousandth of a second. In a typical hard drive, the average seek time is 10 milliseconds. (In the fastest hard drives, the average seek time is 7Ĺ milliseconds; in the slowest hard drives made today, the average seek time is 14 milliseconds; in older hard drives that are no longer made, the average seek time was 28 milliseconds.)

After the head reaches the correct track, it must wait for the disk to rotate, until the correct sector reaches the head.

That rotation time is called the latency. On the average, the head must wait for half a revolution; so the average latency time is a half-revolution. The typical hard drive rotates 90 times per second, so a half-revolution takes half of a 90th of a second, so itís a 180th of a second, so itís about .006 seconds, which is 6 milliseconds.

If you add the average seek time to the average latency time, you get the total average access time. So for a typical hard drive, the average access time = 10 milliseconds seek + 6 milliseconds latency = 16 milliseconds.

During the last few years, hard drive manufacturers have become dishonest: they say the "average access time" is 10 milliseconds, when they should actually say the "average seek time" is 10 milliseconds.

After the head finally reaches the correct sector, you must wait for the head to read the data. If the data consumes several sectors, you must wait for the head to read all those sectors.


Most hard drives for microcomputers are manufactured by three American companies: Seagate Technology (ST), Quantum, and Western Digital.

Seagate was the first of those companies to make hard drives for microcomputers, and it set the standard that the other companies had to follow. New Seagate drives work fine, though Seagateís older models were often noisy and unreliable.

Quantum became famous by manufacturing the hard drives that Apple buys to put in Mac computers. Quantum also builds drives for IBM PC clones. Quantum drives are excellent.

Western Digital has invented hard drives that cost less. Theyíre popular in cheap clones and discount computer stores.

Conner was the first company to invent hard drives tiny enough to fit in a laptop or notebook computer.

Seagate ignored the laptop/notebook market too long, and Connerís popularity zoomed up fast. Conner became the fastest-growing company in the history of American industry! But then Connerís competitors caught up and Connerís popularity sunk back down. In Febuary 1996, Seagate bought Conner.

Other popular manufacturers of hard drives are Americaís Maxtor & Micropolis, Japanís NEC & Fujitsu, and Koreaís Samsung.

When buying a hard drive, you might also need to buy a hard-drive controller.

How many sectors?

Back in the 1980ís, the typical hard-drive controller for IBM-compatible computers put 17 sectors on each track. That scheme was called the Seagate Technology 506 with Modified Frequency Modulation (ST506 MFM).

An improved scheme, which squeezed 26 sectors onto each track, was called the ST506 with Run Length Limited (ST506 RLL). A further improvement, which squeezed 34 sectors onto each track, was called the Enhanced Small Device Interface (ESDI).

Squeezing extra sectors onto each track increases the driveís capacity (total number of megabytes) and also the transfer rate (the number of sectors that the head reads per rotation or per second).

All those schemes ó MFM, RLL, and ESDI ó have become obsolete.

Now the most popular scheme is called Integrated Drive Electronics (IDE). Like ESDI, it squeezes 34 sectors onto each track; but it uses special tricks to transfer data faster.

The original version of IDE was limited to small drives: up to 528M.

A new, improved version, called Enhanced IDE (EIDE), can handle bigger drives. It also goes faster: it transfers 16.6 megabytes per second. Enhanced IDE was invented by Western Digital and is also used by Conner and Maxtor. Seagate invented competing methods (called Fast ATA-2 and Fast ATA-3), which also transfer 16.6 megabytes per second; theyíre used by Seagate, Quantum, Fujitsu, and Samsung.

Those technologies (Enhanced IDE, Fast ATA-2, and Fast ATA-3) are all being replaced by Ultra, which transfers twice as fast: 33.3 megabytes per second! The Ultra version of EIDE is called Ultra IDE; the Ultra version of Fast ATA is called Ultra ATA.

You can buy drives holding up to 8 gigabytes that use those technologies: Enhanced IDE, Fast ATA-2, Fast ATA-3, Ultra IDE, and Ultra ATA.

A totally different fast scheme is the Small Computer System Interface (or SCSI, which is pronounced "scuzzy"). Itís used on most Mac hard drives and the biggest IBM-compatible hard drives.

The newest, fastest version of SCSI, called Ultra Wide SCSI, can transfer 40 megabytes per second. You can buy Ultra Wide SCSI drives that hold up to 23.3 gigabytes.

Discounts on drives

You can buy hard drives cheaply from these discount dealers:

Dirt Cheap Drives

usually lowest prices; ships just to USA; open weekdays 8AM-8PM, Saturday 9AM-3PM, Central Time

3716 Timber Drive, Dickinson TX 77539, phone 800-473-0960 or 281-534-4140


low prices; ships to USA & Canada; open weekdays 8AM-8PM, Saturday 9AM-3PM, Central Time

2201 Pine Drive, Dickinson TX 77539, phone 800-786-1185 or 281-534-3919

Insight (and its division called Hard Drives International)

slightly higher prices; ships to all countries; open always (every day & night, round-the-clock, 24 hours)

1912 W. Fourth St., Tempe AZ 85281, phone 800-INSIGHT or 602-902-1176

IBM-compatible drives Modern, popular IBM-compatible hard drives are IDE, hold 4, 5, or 6 gigabytes, and cost about $60 per gigabyte. For example:

A 4-gigabyte drive costs about $240.

A 5-gigabyte drive costs about $300.

A 6-gigabyte drive costs about $360.

Besides buying the hard drive, you must also buy a card to put in the computerís slot (unless your computer contains such a card already). For example, here are the prices charged by Dirt Cheap Drives:

Capacity Seek Rotation Cache Transfer rate (& why) Brand Model number Price

1.2 G 10 ms 5400 rpm 128 K 16.6 M/sec (IDE) Fujitsu M 1636T $125+$30

1.7 G 10 ms 5400 rpm 128 K 16.6 M/sec (IDE) Fujitsu MPA30 17A $148+$30

2.6 G 10 ms 5400 rpm 128 K 16.6 M/sec (IDE) Fujitsu MPA30 26A $175+$30

3.5 G 10 ms 5400 rpm 128 K 33.3 M/sec (Ultra IDE) Fujitsu MPA30 35U $198+$50

4.3 G 10 ms 5400 rpm 128 K 33.3 M/sec (Ultra IDE) Quantum FB3 4320A $249+$50

5.2 G 10 ms 5400 rpm 128 K 33.3 M/sec (Ultra IDE) Fujitsu MPA30 52U $309+$50

6.4 G 10 ms 5400 rpm 128 K 33.3 M/sec (Ultra IDE) Quantum FB3 6480A $346+$50

7 G 10 ms 5200 rpm 256 K 33.3 M/sec (Ultra IDE) Maxtor M8 7000D $364+$50

8.4 G 10 ms 5200 rpm 256 K 33.3 M/sec (Ultra IDE) Maxtor M8 8400D $427+$50

9 G 9 ms 7200 rpm 512 K 40M/sec(UltraWideSCSI) Micropolis MC 3391W $875+$249

23.2 G 14 ms 5400 rpm 512 K 40M/sec(UltraWideSCSI) Seagate ST4 23451W $1859+$249

In that chart, "G" means "gigabytes". (When discussing hard drives, a "gigabyte" is defined to mean "1000 megabytes".)

At the end of the model number, an A or T means AT-bus IDE, a D or U means double-speed ("Ultra") IDE, and a W means Wide SCSI.

For Fujitsu MPA30 drives, if you multiply the number of gigabytes by 10, you get the model numberís main part. For example, Fujitsuís 5.2-gigabyte drive is called "model MPA30 52U". For drives by Quantum, Maxtor, Seagate, and Western Digital, the model numberís main part is the number of megabytes (approximately). For example, Quantumís 4320-megabyte drive is called "model FB3 4320A".

A cable runs from the drive to a card, which fits into an IBM AT slot.

For a drive thatís Enhanced IDE (or Fast ATA-2 or Fast ATA-3), the best kind of card is an IDE accelerator, which costs $30 ($35 minus $5 discount). For Ultra IDE drives, get an Ultra IDE card instead, which costs $50. For Ultra Wide SCSI drives, get an Ultra Wide SCSI controller card instead, which costs $249. In the table above, the "+$33" or "+$50" or "+$199" is for those cards.

The driveís cache (or buffer) is RAM chips holding copies of the sectors you used recently ó so if you want to look at those sectors again, you can read from the RAM chips (which are fast) instead of waiting for the disk to spin (which is slow).

Those were the prices in November 1997. By the time you read this book, prices might be even lower! Prices continually drop.

Mac drives The price of a Mac hard drive depends on whether the drive is internal (fits inside the Mac) or external (comes in a separate box that you put next to the Mac). Internal drives are cheaper; but if your Mac is small or filled up, you must buy an external drive instead.

For Mac drives, here are Megahaus prices:

Capacity Seek Rotation Cache Brand Model Internal External

2.1 G 10 ms 5400 rpm 128 K Quantum FB3 2160S $225 $275

3.2 G 10 ms 5400 rpm 128 K Quantum FB3 3240S $275 $325

4.3 G 10 ms 5400 rpm 512 K Micropolis MC 4743 $325 $375

6.4 G 10 ms 5400 rpm 128 K Quantum FB3 6480S $405 $455

9 G 9 ms 7200 rpm 512 K Micropolis MC 3391 $850 $910

23.2 G 13 ms 5400 rpm 512 K Seagate ST4 23451W $1689 $1839

Buy a big drive Back in the 1980ís, a 40-megabyte drive was considered "big", and it was the most popular size to buy.

In the 1990ís, a 40-megabyte drive is considered "too small". Even a 540-megabyte drive is considered "too small". Hereís why:

To use modern programs on an IBM-compatible computer, you must make sure the hard disk contains some fundamental software, called MS-DOS and Windows. The newest version of MS-DOS (version 6.22) consumes over 5 megabytes. Windows 3.11 consumes over 10 megabytes. That makes 15 megabytes altogether! (Windows 95 consumes even more: just the "important" parts of Windows 95 consume over 35 megabytes!)

The typical Windows word-processing program (such as such Word Perfect 6.1 or Microsoft Word 6) consumes about 25 megabytes. So altogether, for DOS plus Windows plus a word-processing program, youíve consumed over 40 megabytes already!

Youíll need additional megabytes for additional business programs (about 10 megabytes per program), plus additional megabytes to hold what you type. After buying the computer, youíll probably spend the next several years accumulating many programs (a few each year). After a year or two, youíll accumulate over 540 megabytes, and youíll wish youíd bought a bigger drive instead.

Buy at least a 2-gigabyte drive. A 2-gigabyte drive costs just slightly more than a 1-gigabyte drive and will last you for many years. Youíre buying peace of mind!

Itís much cheaper to buy a 2-gigabyte-megabyte drive now than to buy a 1-gigabyte-megabyte drive now and another 1-gigabyte-megabyte drive later. Another reason for buying a 2-gigabyte drive is that it will act faster than a 1-gigabyte drive.

For example, suppose you want to store 1 gigabyte of info, and youíre debating whether to buy a 1-gigabyte drive or a 2-gigabyte drive. Suppose each drive is advertised as having a 10-millisecond seek time. The 2-gigabyte drive will nevertheless act faster. Hereís why:

Suppose you buy the 2-gigabyte drive and use just the first gigabyte of it. Since youíre using just the first half of the drive, the head needs to move just half as far as usual; so over the 1-gigabyte part that youíre using, the effective average seek time is just half as much as usual: itís 5 milliseconds!


If you need more than 23.3 gigabytes, attach several hard drives together. The group of drives is called a drive array and acts as one huge drive. That technique is called RAID (which originally stood for Redundant Array of Inexpensive Disks but now stands for Redundant Array of Independent Disks).

Some RAID versions make some of the drives automatically hold backup copies of the other drivesí data, so if a drive fails you can recover the data.


Instead of buying a program on a floppy disk, you can buy a program on the same kind of compact disk (CD) that holds music.

A CD that holds music is called a music CD (or audio CD).

A CD that holds computer data instead is called a computer CD (or data CD). Since the computer data on it cannot be erased, a computer-data CD is also called a CD read-only memory (CD-ROM).

To make your computer read the CD-ROM disk, put the disk into a CD-ROM drive, which is a souped-up version of the kind of CD player that plays music.

Like an ordinary CD player, a CD-ROM drive uses just optics. No magnetism is involved. The drive just shines a laser beam at the shiny disk and notices, from the reflection, which indentations (pits) are on the disk. The pattern of pits is a code that represents the data. So a CD-ROM drive is an example of an optical disk drive.

To put the disk into the drive, press a button on the drive. That makes the drive stick its tongue out at you! The tongue is called a tray. Put the disk onto the tray, so that the diskís label is face-up. (If the drive is old-fashioned, you must put the disk into a caddy first; but the most modern drives are caddyless.) Then push the tray back into the drive. Finally, use the keyboard or mouse to give a command that makes the computer taste what youíve put on its tongue.

IBM drive letters

Hereís how a modern IBM-compatible computer assigns the drives:

Drive A is a 3Ĺ-inch floppy drive (1.44M).

Drive B is a 5ľ-inch floppy drive (1.2M).

Drive C is a hard drive (about 4G).

Drive D is typically a CD-ROM drive.

But if your computer has two hard drives, hereís what happens: the first hard drive is C, the second hard drive is D, and the CD-ROM drive is the next letter (E).

If you bought just one hard drive but plan to buy a second hard drive later, you can leave "drive D" empty and make the CD-ROM drive be E.


CD-ROM disks come in two sizes:

The standard size has a diameter of 12 centimeters (which is about 5 inches) and holds 650 megabytes.

The miniature size has a diameter of 8 centimeters (which is about 3 inches) and holds 180 megabytes.

Your CD-ROM drive can handle both sizes of CD-ROM disks. Each CD-ROM disk is single-sided: all the data is on the diskís bottom side ó the side that doesnít have a label.

On a standard-size CD-ROM disk, there are 2 billion pits. Theyíre all arranged into a single spiral (like the groove on a phonograph record). If you were to unravel the spiral, to make it a straight line, it would be 3 miles long!

On a CD, each "song" is called a track; it can hold music or computer data. Each "song" (track) can be as long or as short as you wish.

The standard-size CD can hold 99 tracks, totaling an hour of music (for an audio CD) or 650 megabytes (for a CD-ROM disk).

Yes, a standard-size CD-ROM disk holds 650 megabytes, which is a lot!

Itís about 450 times as much as a high-density 1.44M floppy.

Itís about 1800 times as much as a 360K floppy.

Since a CD-ROM disk holds so much, a single CD-ROM can hold a whole library (including encyclopedias, dictionaries, other reference materials, famous novels, programs, artwork, music, and videos). Itís the ideal way to distribute massive quantities of information! Moreover, a CD-ROM disk costs just $1.50 to manufacture (once youíve bought the appropriate CD-ROM-making equipment, which costs several thousand dollars).

CD-ROM disks store info differently than floppy & hard disks:

On a CD, each track is part of a spiral.

On a floppy disk or hard disk, each track is a circle.

On a CD, different tracks have different lengths and hold a different number of bytes.

On a typical floppy disk or hard disk, all tracks have the same number of bytes as each other.


When buying a CD-ROM drive, the most important factor to consider is the driveís speed.

Transfer rate The speed at which the drive spins is called the transfer rate. The higher, the better!

On the first CD-ROM drives that were invented, the transfer rate was the same speed as music CDís: 150 kilobytes per second.

Then came drives that could spin twice as fast: 300 kilobytes/second. Thatís called double speed or 2X.

Then came drives that could spin thrice as fast: 450 kilobytes/second. Thatís called triple speed or 3X.

Then came drives that spun four times as fast: 600 kilobytes/second. Thatís called quad speed or 4X.

Then came 4ĹX drives, then 6X, then 8X, then 10X, then 12X. During the summer of 1997, most drives sold were 12X, which transfers 1800 kilobytes/second.

Now most drives are even faster! The fastest drives are called 24X/12X (or 24X maximum or 24X max): their outer tracks are read at a maximum speed of 24X, though their inner tracks are read at just 12X.

Seek time The average time it takes for the head to move to the correct track is called the average seek time.

The lower the average seek time, the better!

Under 100 milliseconds is great.

100-200 milliseconds is okay.

200-300 milliseconds is disappointing.

Over 300 milliseconds is terrible.

Buying a drive

Here are the cheapest good drives:

Transfer rate Seek Buffer Brand Model Price

12X 150 ms 256K Cyber Cyber 12X $69

16X max 90 ms 256K Toshiba XM 6002B $79

24X max 85 ms 256K Toshiba XM 6102B $89

Each of those drives is IDE. You can get those prices from these discount dealers:

Dirt Cheap Drives

3716 Timber Drive

Dickinson TX 77539

800-473-0960 or 281-534-4140

Harmony Computers & Electronics

1801 Flatbush Ave.

Brooklyn NY 11210

800-441-1144 or 718-692-3232

Those prices are for just the bare drive; add $30 for an installation kit.

If you already own a CD-ROM drive thatís at least 2X, donít bother replacing it by a faster one. Faster drives make most CD-ROM program run just slightly faster, since most CD-ROM programs are still designed under the assumption each CD-ROM drive is just 2X.

External drives Those prices are for internal drives, which fit inside the computerís system unit. If your system unit is filled up and doesnít have any room left to insert an internal drive, you must buy an external drive instead, which sits outside the system unit and costs about $30 more.

Multimedia kits If you buy a CD-ROM drive, youíll also want a sound card, a pair of stereo speakers, and a few sample CD-ROM disks (so you can admire all that equipment you bought). That combo ó a CD-ROM drive, sound card, pair of speakers, and sample CD-ROM disks ó is called a multimedia kit.

The most popular 24X-max multimedia kit is the Sound Blaster Discovery AWE64 24X, made by Creative Labs. You can get it for just $239 from discount dealers such as Harmony.

These multimedia kits cost less:

CD-ROM speed Brand Price of entire multimedia kit

8X Hi Val $140

12X Hi Val $160

16X max Hi Val $170

20X max Hi Val $190

24X max New Com $200

Those prices are from a discount dealer called USA Flex (444 Scott Dr., Bloomingdale IL 60108, 800-USA-FLEX or 708-582-6206).

Tough installation

Warning: CD-ROM drives (and multimedia kits) are extremely difficult to install correctly because they tend to conflict with other hardware and software. Even if you follow the instructions in the setup manual, the stuff typically doesnít work.

If you try to get help by phoning the company that made the multimedia kit (such as Creative Labs), you almost always get a busy signal. Even if you finally get through to a technician, the technician canít help you much, since the technician doesnít know enough details about the other hardware and software your computer contains.

Most consumers give up in disgust, return the stuff to the store, and pay the store about $70 to do the installation.

Some stores offer "free" installation ó but do so by advertising a higher price for the CD-ROM drive.

When you buy a new computer, you can ask the salesperson to include a multimedia kit and install it. A computer that includes an installed multimedia kit is called a multimedia computer system.

Caring for your CD-ROM disks

A CD-ROM diskís main enemy is dirt.

When you buy a CD-ROM disk, it comes in a clear square box, called the jewel box. To use the CD-ROM disk, remove it from the jewel box and put the disk into the drive. When you finish using the disk, put it back into the jewel box, which keeps the dust off the disk.

When putting the CD-ROM disk into or out of a drive, donít put your fingers on the diskís surface: instead, hold the disk by its edge, so your greasy fingerprints donít get on the diskís surface.

Once a month, gently wipe any dust off the CD-ROM diskís bottom surface (where the data is). While wiping, be gentle and donít get your greasy fingerprints on the disk. Start in the middle and wipe toward the outer edge.

For example, my assistant and I were getting lots of error messages when using a sample CD-ROM disk we bought from Microsoft. I was going to phone Microsoft to complain, but my assistant asked, "What about dust?" I flipped the CD-ROM disk over and sure enough, a big ball of dust was on the diskís bottom side, where the data is recorded. I wiped it off. That CD-ROM disk has worked perfectly ever since.

I was so embarrassed! If my assistant hadnít reminded me to wipe the dust off, Iíd have wasted hours of Microsoftís time hunting uselessly for a high-tech reason my CD-ROM disk wasnít working.

Donít put any fluids on the disk. The fluids that clean phonograph records will wreck CD-ROM disks.

If you want to write on the disk, use a felt-tipped pen (not a ballpoint or pencil). Donít stick any labels on the disk.

The typical CD-ROM disk will last about 12 years. Then the aluminum on its surface will start to oxidize (corrode), and the CD will become unreadable.


You can create your own CDís, in the privacy of your home, if you buy a CD-Recordable drive (CD-R drive).

The cheapest popular CD-R drive is the Sure Store CD-Writer, manufactured by Hewlett-Packard. It costs just $380. Unfortunately, like most CD-R drives, it writes onto the CD slowly (at a speed of 2X), reads from the CD slowly (6X), and requires that you buy a SCSI card. It also requires you to buy blank CD-R disks, which are pricey: $3.99 each if you buy 1, $3.49 each if you buy 10, $2.99 each if you buy 30. All those prices are from a discount dealer, USA Flex (800-444-4900).

Although a CD-R drive can write onto a disk, it cannot erase or edit what you wrote. For more flexibility, you can buy a CD-ReWritable drive (CD-RW drive), which can write onto a blank CD and then edit what you wrote.

Unfortunately, CD-RW drives are expensive. The cheapest is made by Ricoh and costs $600. That price includes a SCSI card. The drive writes 2X and reads 6X. It requires you to buy blank CD-RW disks, which are very expensive: $26 each if you buy 1, $24 each if you buy 10, $22 each if you buy 30. Those prices are from USA Flex.


In 1997, the electronics industry began selling an improved kind of CD, called a Digital Versatile Disk (DVD). It looks like a standard-size CD but holds more info.

Unlike a standard CD, which holds just an hour of music or 650M of data, a standard DVD can hold a 2-hour movie (including the video and sound) or 4.7G of data. Since it can hold a movie, some movie lovers call it a "Digital Video Disk", but itís more versatile than just that!

A DVD can be recorded on just the bottom side (like a CD) or on both sides. (To use the second side, you must remove the disk from the drive and flip the disk upside down, like youíd flip a phonograph record.) A dual-sided DVD can therefore hold 9.4G of data.

An improved technology, called dual-layer DVD, puts nearly two layers of data on each side, so you get 8.5G per side, 17G total.

A DVD that contains computer data (instead of a movie or music) is called a DVD-ROM disk. To use it, put it in a DVD-ROM drive, which costs about $500.

Your DVD-ROM drive can read DVD-ROM disks and also standard CD-ROM disks. Unfortunately, just the fanciest DVD-ROM drives can also read CD-R and CD-RW disks.

To create your own DVDs in your own home, you can buy a DVD-Recordable drive (DVD-R drive), which costs about $10,000. To create and edit your own DVDs, you must buy a DVD-RAM drive, which costs even more.