A storage device for which restoration of data is possible without the
need for excess equipment includes mirrored storage units for online use
and for backup, and is capable of performing a data backup while
continuing input and output of data. The storage device is further
provided with an update data storage unit for storing update data.
According to the storage device, even if the information stored in a
storage unit for online use is not capable of being read because of a
fault in the storage unit, it is possible to restore the data in the
backup storage unit to the state that existed in the storage unit for
online use immediately before the fault by applying the update data to
the information stored in the storage device for backup.