OPTIMIZING METHOD FOR BUFFER RATIO

PURPOSE:To provide a means to improve efficiency for using a bus by changing the value of a buffer ratio in an external storage device. CONSTITUTION:This control method of the external storage device adopts a zone bit recording system to designate the buffer ratio for controlling the occupying time of a bus 16 in the case of an input/output processing from a host device 11, to perform access, to divide a magnetic disk medium 13 into plural zones in a circumferential direction and to change write density for each ratio and by changing the buffer ratio in the external storage device corresponding to the zone storing accessed data, data transfer is continuously executed.