Embodiments of the invention raise the performance of a HDD by controlling the timing of notifying of command completion. In one embodiment, transmission of command completion notifications to a host is managed by a host interface manager. If two data write addresses respectively for two queued commands are adjacent or near to each other, that is, these addresses on the magnetic disk can be accessed without rotational latency, the host interface manager postpones the transmission of a command completion notification (X) concerning the first write command (X). Two command completion notifications (X) and (Y) are performed at a time after the write data (Y) for the next command is transmitted and its write to the medium is completed.