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 . This is computed by averaging the time necessary for all possible seeks. This tends to overestimate actual average seek time because of locality of disk references. The time necessary for the requested sector to rotate under the head. On average, the disk must rotate halfway around to get to a desired sector.