How Hard Disks Work
All hard disks are constructed similarly. A central spindle supports one or more platters, which are thin, flat, circular objects made of metal or glass, substances chosen because they are rigid and do not expand and contract much as the temperature changes. Each platter has two surfaces, and each surface is coated with a magnetic medium. Most drives have multiple platters mounted concentrically on the spindle, like layers of a cake. The central spindle rotates at several thousand revolutions per minute, rotating the platters in tandem with it.
A small gap separates each platter from its neighbors, which allows a read-write head mounted on an actuator arm to fit between the platters. Each surface has its own read-write head, and those heads “float” on the cushion of air caused by the Bernoulli Effect that results from the rapid rotation of the platter. When a disk is rotating, the heads fly above the surfaces at a distance of only millionths of an inch. The head actuator assembly resembles a comb with its teeth inserted between the platters, and moves all of the heads in tandem radially toward or away from the center of rotation.
Platters are cheaper than heads. That means some drives have an odd number of heads, leaving one surface unused. For example, Seagate Barracuda 7200.7 series drives use 40 GB/surface technology and are available in 40, 80, 120, and 160 GB models. The 40 and 80 GB models use one platter with one and two heads, respectively. The 120 ...