An execution queue stores a write command from the host in response to issuance of the write command from the host, and is removed from the execution queue in response to a signal indicating that data designated by the write command has been written to the hard disk. A holding queue stores the write command removed from the execution queue. In response to the command being stored in the holding queue, a request is issued for an acknowledgment from the host. The write command is removed from the holding queue in response to the acknowledgment being received from the host. An outgoing queue stores the write command removed from the holding queue for deletion. The queues are controlled by queue management hardware, the request is issued by the queue management hardware, and the signal and acknowledgment are received by the queue management hardware.