A disk drive consists of a disk pack containing one or more platters stacked like phonograph records.
Information is stored on both sides of the platter.
Each platter is divided into concentric rings called tracks , and each track is divided into sectors . All transfers to and from the disk are performed at the sector level. For example, to modify a single byte, the system reads the sector containing the byte off the disk, modifies the byte, and rewrites the entire sector. The operation of moving the head from one track to another is called seeking , and the heads generally move together as a unit. Because the disk heads move together as unit, it is useful to group the tracks at the same head position into logical units called cylinders .