Hard Disk = one or more disks in a rectangular metal box spinning at up to about 15,000 revolutions per minute. The disks themselves are made of glass or aluminum and are coated with magnetic iron oxide particles (essentially the same substance that is found in audiocassette and videocassette tapes). The bits and bytes (characters) of your data and programs are stored magnetically in sectors and tracks on the surfaces of these disks. The data is read by tiny pick-up heads attached to arms that swing across the surface of the disks in an arc. Hard disks can store anywhere from about 10 Megabytes (million bytes or characters) to 1,000 Gigabytes (billions of bytes or characters) or 1 Terabyte, or more. The major manufacturers of hard disks are Fujitsu, Hitachi, Maxtor, Samsung, Seagate, and Western Digital.
Hard disk (with the top cover removed)