Your hard drive stores the computer's operating system and the software programs you use every day. It also stores your business records, the novel you're writing, your income tax returns, your recipes, saved games, your photos, your address book, your vacation movies and whatever other information you entrust to it. That makes the hard drive the most personal part of the computer. You can always replace the drive if something goes wrong, but you can't replace the information on it. So keep your hard drive healthy and make regular backups.

Where do you find the hard drive?

On most computers, the case conceals the hard drive. But you can also buy an external drive that connects to the computer with a cable. External drives are popular with people who want to use the same hard drive for more than one computer. They're also an option if you want to add a second drive but don't have room inside the computer.

What does a hard drive look like?

If you took a hard drive apart (kids, don't try this at home), you'd see magnetic platters turning at 3,000 to 6,000 revolutions per minute. When your computer needs to get data from the drive, tiny magnetic heads move across the spinning platters and read the information as binary ones and zeroes. The drive heads record data in the same manner when you store something on your hard disk. You can hear the drive clicking when this happens, and, on some computers, a little blinking light tells you that the drive is working.

How much does a hard drive hold?

Hard drive storage capacity is measured in megabytes or gigabytes. A megabyte is the equivalent of a million characters of text -- about the length of an average novel. A gigabyte is a billion characters, or a thousand novels. Older computers may have hard drives that store 20 gigabytes or less. Today's computers typically come with drives that can store 180 gigabytes of information or more.

A gigabyte may seem like a lot of storage, but modern operating systems can eat up 3 gigs of space. A word processor or spreadsheet can easily use 1 gig, and games that come on CD-ROMs often want to drop 500 to 800 megabytes' worth of files on your hard disk. Many new computers arrive with a big chunk of their hard drive's space already filled with bundled OEM or proprietary software. So if you use your computer a lot, even a 200-gigabyte drive won't seem all that roomy after a while.

What kind of controller does it use?

Storage capacity aside, several features distinguish one model of hard drive from another. Chief among these is the hard-drive controller, a circuit board that handles the flow of information between the hard drive and the computer. If you're replacing or adding a drive, you'll need to know what kind of controller your computer uses.

Most IBM-compatible computers use IDE or enhanced EIDE controllers . The abbreviation stands for "Integrated Drive Electronics," which means the controller is part of the drive itself. IDE drives require only a simple connection inside the computer. The major drawback of IDE drives is a limit of two hard disks per machine. But newer SATA interfaces can handle up to four hard drives or more. Most new computers have the SATA adapter built right onto the main circuit board.

Macintosh computers and some heavy-duty PCs use SCSI drives. SCSI is pronounced "scuzzy" and stands for "Small Computer Systems Interface." SCSI drives require a sophisticated controller board inside the computer. SCSI's main advantage is that one controller can handle up to seven hard drives or other devices, like scanners or CD-ROM drives.

How fast is the hard drive?

The faster your hard drive, the better your computer will perform. The most common measure of speed is "seek time," the average time it takes the drive heads to move between tracks on a platter. Most drives packaged with today's computer systems have seek times in the 10-12 millisecond range (the lower the number, the faster the drive). However, other speed measurements are just as important, such as the data transfer rate -- how fast the drive actually dumps information into the computer.