1. A magnetic disk device comprising: a disk that includes a plurality of tracks and magnetically stores therein data; a magnetic head that reads and writes data on the tracks; a voice coil motor that moves the magnetic head to select an arbitrary track from the tracks; a read/write channel that modulates the data read and written by the magnetic head; a servo control circuit that controls a position of the tracks; a controller that includes a micro processing unit and performs control; an error check and correct circuit that corrects the data; an interface circuit that connects the magnetic disk device to an external device; a data writing unit that classifies sectors along one track into a plurality of sector groups and writes to a physical address of each of the sector groups data of a logical address that is different from a logical address corresponding to the physical address in same track; and a first nonvolatile memory that stores therein a conversion table of the logical address and the physical address.