Disk is divided into many circular pathes when it is formated and these circular pathes are called track. From outside to inside, tracks are orderly numbered from 0. The number of tracks is 300-1024 in each side of a disk. The new disk of large capacity has more tracks in each side. As shown in the picture, the information is recorded in the form of pulse train. Those concentric circles do not continuously record information, but are divided into arcs. Because of different radial lengths, angular velocities and liner velocities (the speed of outer circles is higher than that of inner circles), the movement length of outer circles is longer than that of inner circles in the same revolving speed and time period. Every arc is called sector numbered from 1. And data in every sector is considered as a whole to be read or write. Usually there are hundreds or thousands tracks in one side of a standard hard disk of 3.5 inches. These tracks are invisible and they are just some areas magnetized in specific form in disk. These tracks have been planed when the disk is formated.