Modern HDDs typically contain multiple platters, all of which are mounted on a single shaft, in order to maximize the data storage surface in a given volume of space. Each platter is usually made from a thin sheet of aluminum or glass. The magnetic coating is generally a ferric oxide or a cobalt alloy, which is, in turn, covered by a thin layer of a very hard material and, finally, by a thin layer of lubricant.
The magnetic media on each side of each platter is divided into a series of tracks. A track is any of the concentric circles over which one magnetic head passes while it is stationary but the platter is rotating at high speed. A magnetic head, also commonly referred to as just a head, is a small, high-sensitivity electromagnet that used for reading and writing data on the magnetic media. Most platters require two heads, one for each side.
Each track, in turn, is divided into sections called sectors. A sector generally contains 512 bytes for data storage and is the smallest unit of data that can be accessed by a disk drive (although software makes it possible to access individual bytes and even individual bits).
Created March 16, 2006. Updated May 21, 2006.