In a magnetic disk apparatus which is removably mounted or embedded to a host
apparatus and intermittently accessed for recording/reproducing data, a RAM contained
in the host apparatus is utilized for a portion or entirety of a buffer RAM for
use in the intermittent access to accomplish a reduction in size and cost of the
magnetic disk apparatus. In addition, the magnetic disk apparatus is controlled
to rotate a magnetic disk at a variable rotational speed, and operated at a rotational
speed at which the total power consumption is minimized in accordance with the
capacity of the utilized buffer RAM and whether an access is made for recording
or reproduction, thereby improving a power saving effect.