Sector

A sector is normally the smallest individually addressable unit of information stored on a hard drive platter, and usually holds 512 bytes of information. Sectors are numbered sequentially starting with 1 on each individual track. Thus, Track 0, Sector 1 and Track 5, Sector 1 refer to different sectors on the same hard drive. The first PC Hard discs typically held 17 sectors per track. Today, they can hold thousands of sectors per track..”