Number of sectors per track
(more sectors on the outside as apposed to the tracks near the spindle,)
ATA/IDE drives were much more efficient at storing data and have replaced the now archaic MFM and RLL drives.
They use zone bit recording (ZBR), where the number of sectors dividing each track varies with the location of groups of tracks on the surface of the platter.
Tracks nearer to the edge of the platter contain more blocks of data than tracks close to the spindle,because there is more physical space within a given track near the edge of the platter.
Thus, the CHS addressing scheme cannot correspond directly with the physical geometry of such drives, due to the varying number of sectors per track for different regions on a platter. Because of this, many drives still have a surplus of sectors (less than 1 cylinder in size) at the end of the drive, since the total number of sectors rarely, if ever, ends on a cylinder boundary.
See MFM and RLL drives and below ATA/IDE drives:- See History