A CPU determines whether or not the disk access range designated by a
read/write command from a host contains a defective sector. If the disk
access range contains a defective sector, the CPU suspends disk access
when a head has reached the defective sector. In this state, the CPU
activates a buffer access circuit. The buffer access circuit copies data
between an area of an alternate sector buffer and data buffer. The area
of the alternate sector buffer stores data corresponding to the data
written in the alternate sector allocated to the defective sector reached
by the head.