When you save data or install programs on your computer, the information is typically written to your hard disk. The hard disk is a spindle of magnetic disks, called platters, that record and store information. Because the data is stored magnetically, information recorded to the hard disk remains intact after you turn your computer off. This is an important distinction between the hard disk and RAM, or memory, which is reset when the computer’s power is turned off.

The hard disk is housed inside the hard drive, which reads and writes data to the disk. The hard drive also transmits data back and forth between the CPU and the disk. When you save data on your hard disk, the hard drive has to write thousands, if not millions, of ones and zeros to the hard disk. It is an amazing process to think about, but may also be a good incentive to keep a backup of your data.

A hard disk is part of a unit — often called a disk drive, hard drive or hard disk drive — that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today’s computers typically come with a hard disk that can contain anywhere from billions to trillions of bytes of storage.

A hard disk is actually a set of stacked disks, like phonograph records. Each disk has data recorded electromagnetically in concentric circles, or tracks, on the disk. A head, similar to a phonograph arm but in a relatively fixed position, writes or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, an operation called a seek. Data already in a disk cache, however, will be located more quickly.

It consists of several major components inside its casing. These include the platter for storing data, a spindle for spinning platters, a read/write arm for reading and writing data, an actuator to control the action and movement of the read/write arm and a logic board.

It include one or more aluminum, glass or ceramic platters made of substrate material with a thin magnetic surface, or media layer, to store data. Platters store and organize data in specific structures — tracks, sectors and clusters — on this media layer, which is only a few millionths of an inch thick. A superthin protective and lubricating protective layer above the magnetic media guards against accidental damage and contamination by foreign material, like dust.