An image forming apparatus (storage system) has a first and a second HDD, and a temporary storage unit. The apparatus divides input data into a first and a second divided data pieces and writes the first and the second divided data pieces in the temporary storage unit. The apparatus transfers the written first divided data piece to the first HDD and transfers the written second divided data piece to the second HDD. The apparatus copies the transferred first and second divided data pieces into the temporary storage unit. The apparatus makes a backup of the copied first divided data piece in the second HDD, and makes a backup of the copied second divided data piece in the first HDD.