Embodiments of the invention achieve data write in an appending manner by conversion from a logical block address to a physical block address in a HDD that has only one storage device and does not have a large-scale cache memory. In one embodiment, a check is made as to whether or not the size of an address translation table in a cache memory exceeds a threshold value. If the size exceeds the threshold value, entries whose number is specified are selected by the LRU method. The selected entries are added to a WRITE buffer, and the address translation table is saved on the HDD by executing WRITE. Seek time of a head at the time of WRITE is reduced, thereby improving WRITE performance. There is produced an effect of building such a snapshot that while a usual access to a HDD volume is allowed, it is possible to make an access to a volume of the snapshot which is a past state of the HDD. Disabling write after writing to the HDD is disabled. Thus, it possible to roll back to a HDD state at an arbitrary point of time.