Storage device and data backup method

A storage device for backing up data to an another storage device periodically comprising:

• a storage unit for storing data;
• a memory for storing management information indicative of locations of updated portion of the data;
• a local storage unit for storing differential data as backup data;
• a controller for controlling the storage device to back up data to the another storage device in accordance with a process comprising the steps of:
• updating data stored in the storage unit;
• updating management information corresponding to the updated portions of the data;
• transmitting differential data to the another storage device, the differential data being the updated portions of the data which have been updated after preceding backing up of data until current backing up of data;
• resetting the management information after transmitting the differential data;
• storing, when the storage device fails transmission of the differential data to the another storage device, the differential data and the associated management information in the local storage unit;
• and
• transmitting the differential data to the another storage device at a later time after resetting of the management information.