Each storage unit is provided with a table for storing a corresponding unit ID and count value. The controller receives a formatting instruction specifying a first unit ID, and updates the count value on a table corresponding to the first unit ID. The controller receives a write command specifying a second unit ID, acquires a count value corresponding to the second unit ID from the table, and attaches the count value to the data, and writes the data to the storage unit. When a read command specifying the second unit ID is received, the controller reads the data from the storage unit, acquires the count value corresponding to the second unit ID from the table, compares this count value and the count value attached to the read data, sends the read data to the transmission source of the command if these values match, and does not send the data to the transmission source of the command if these values do not match.