Patent ID: 7000134

Claim:
A disk drive device comprising: a speed control circuit for controlling the rotation speed of a disk installed in said disk drive device; an external power source connection detecting circuit for detecting connection of said external power source; a mode setting circuit for selectively establishing, in accordance with a detection output from the external power source connection detecting circuit, a maximum speed enabling mode in which the maximum disk rotation speed which can be achieved by a host device is not restricted or a maximum speed restricting mode in which said maximum disk rotation speed is restricted; and a buffer memory for temporarily storing data transferred from the host device; wherein, during data recording onto the disk, said mode setting circuit sets the rotation speed of the disk at the rotation speed which can be achieved in the maximum speed restricting mode when said external power source connection detecting circuit is unable to detect that the external power source is connected, and the possibility of buffer underrun in said buffer memory is detected because the speed of transfer of data from the host device is less than the speed of reading of data from said buffer memory.