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The hard disk drive (HDD) is one of the most important and yet mysterious parts of a computer system. HDDs are sealed units used for nonvolatile data storage. Nonvolatile, or semipermanent, storage means that the storage device retains the data even when no power is supplied. Because HDDs store crucial programming and data, the consequences of failures are usually serious. To build, maintain, service, or upgrade a PC system properly, it is important to know how hard disks function.
HDDs contain rigid, circular platters, usually constructed of aluminum or glass (see Figure 8.16). These platters can’t bend or flex—hence the term hard disk. In most drives you can’t remove the platters, which is why they are sometimes called fixed disk drives.
Figure 8.16 Hard disk heads and platters.
Although hard disk drives continue to be the most important mass storage devices used in personal computers, many high-performance desktops and smaller (under 15-inch screen size) laptops use a flash memory-based mass storage device known as a solid-state drive (SSD) instead of an HDD. SSDs are much faster than HDDs because they use flash memory rather than mechanical parts. To learn more about SSDs, see Chapter 9, “Flash and Removable Storage.”