Magnetic disk device and control method of magnetic disk device

A control method of a magnetic disk device comprising:

• dividing a predetermined recording area constituted of a plurality of cylinders adjacent to each other in magnetic disks into a first area which is specified in such a manner as to correspond to a defect existing in the predetermined recording area and to which no logical addresses are allocated, and a second area which is an area other than the first area and to which logical addresses are uniquely allocated;
• allocating, when the number of defects is less than a first threshold, logical addresses in such a manner that a head which carries out read/write of data in the second area sequentially makes access to sectors of the second area;
• carrying out, when the number of defects is greater than or equal to the first threshold and is less than a second threshold greater than the first threshold, read/write of data from/to tracks ranging up to a track positioned on this side of the first area and thereafter allocating logical addresses in such a manner that the head is changed to another head and read/write of data is carried out by the new head;
• and changing, when the number of defects is greater than or equal to the second threshold, a position of the sector from which allocation of logical addresses is to be started in the second area according to a position of the defect and thereby allocating logical addresses.