of the scsi protocol controller 705 the hard disk controller controls a servo controller 707 and a read write channel 708 based on the received instruction and executes writing or reading data to and from a storage medium the servo controller 707 controls a voice coil motor 714 so as to move a head 711 mounted on a tip of an actuator arm 713 to a position on a sector to be read of the storage medium 712 when the head 711 has moved to the sector to be read out the read write channel 708 executes sending and receiving data to and from the head 711 at this time the data saved in the cache memory 703 is written to the storage medium when writing data and the data saved in the storage medium is written to the cache memory 703 when reading data when the power of the disk drive is turned on the memory controller and microprocessor 704 read a start program from the flash rom 701 then the memory controller part initializes the main memory 702 and cache memory 703 controls the servo controller 707 and read write channel 708 by way of the hard disk controller and scsi protocol controller 705 further.