Disk storage device and cache control method for disk storage device

A disk storage device stores write data in a cache area of a data buffer according to a write command from a host, reports write completion to the host, and then writes the write data in the cache area to a storage medium. The disk storage device stores the write data stored in the data buffer in a save area created on the storage disk in response to the reception of a command which requires integrity, other than a write command, and reports a response to the host. Therefore processing time of this command and response time to the host can be decreased, while maintaining the safety of the write data.