The hard drive

A hard drive, known as a hard disk, is a common form of mass storage for com- puter data. The drive consists of several disks, called platters, arranged in a stack. They are made from aluminum or other rigid material, coated with a ferromagnetic substance similar to that used in audio or video tape. The platters are spaced a frac- tion of an inch apart. Each of them has two sides and two read/write heads. The assembly is enclosed in a sealed cabinet.

Drive action
When computer is switched off, the hard drive mechanism locks heads in a position away from platters. This prevents damage to heads and platters if the computer is moved. When computer is powered up, the platters spin at several thousand revolutions per minute (rpm). The heads hover a few millionths of inch above and below the platter surfaces.When you type a command or click on the icon telling computer to read or write data, hard drive mechanism goes through the series of rapid, complex, and precise movements. The head should be positioned over particular spot on the platter where

Figure—At point A, edgewise view of a hard drive, showing four platters. At point B, broadside view of a platter, showing one track and one sector.

the data is located. Then the head should stabilize its position and generate or detect the magnetic fields. All this occurs in a small fraction of a second