Platters are 1 - 8 inches wide. Surface Each platter has two surfaces, both of which may be used for recording. Track A surface is divided into tracks, which are concentric circles containing data. The set of tracks at corresponding locations on all of the surfaces is called a cylinder. Sector A track is divided into sectors each of which holds a fixed amount of data, usually 512 - 4096 bytes.
Older disks have a constant number of sectors per track.
Newer disks record more sectors on the outer tracks using a constant bit density . Each surface is read and written by its own head. All of the heads in a drive are connected together and move as a single unit. Most disks have just one such unit. The industry standard is to use average seek time .