Definition of a Hard Disk

To many users, the hard disk drive is the most important and yet the most mysterious part of a computer system. A hard disk drive is a sealed unit that a PC uses for nonvolatile data storage. Nonvolatile, or semipermanent, storage means that the storage device retains the data even when no power is supplied to the computer. Because the hard disk drive is expected to retain data until deliberately erased or overwritten, the hard drive is used to store crucial programming and data. As a result, when the hard disk fails, the consequences are usually very serious. To maintain, service, and upgrade a PC system properly, you must understand how the hard disk functions.

A hard disk drive contains rigid, disk-shaped platters, usually constructed of aluminum or glass (see Figure 9.1). Unlike floppy disks, the platters can't bend or flexhence the term hard disk. In most hard disk drives, you can't remove the platters, which is why they are sometimes called fixed disk drives. Removable hard disk drives are also available. Usually, this term refers to a device in which the entire drive unit (that is, the disk unit containing the platters as well as the rest of the drive) is removable, but it can also refer to cartridge drives, where the platters are contained in a removable cartridge.

Figure 9.1. Hard disk heads and platters.

Note

Hard disk drives are sometimes referred to as Winchester drives. This term dates back to 1973, when IBM introduced the model 3340 drive, which had 30MB of fixed platter and 30MB of removable platter storage on separate spindles. The drive was codenamed Winchester by project leader Ken Haughton because the original capacity designation (30-30) sounded like the popular .30-30 (caliber-grains of charge) cartridge used by the Winchester 94 rifle introduced in 1895. The original 3340 "Winchester" drive was the first to use a sealed head/disk assembly, and the name has since been applied to all subsequent drives with similar technology.