The control unit calculates the life difference resulting from the subtraction of the remaining life of existing devices that are the existing memory devices from the remaining life of an added device that is the added memory device and, if the life difference is smaller than the threshold, newly distributes and stores the data stored in the existing devices and the added device, among and in existing devices that are larger in number than the existing devices and the added device, by a second data unit greater than a first data unit each time. On condition that the life difference is thereby made smaller than the threshold, the capacity usage rates of the existing devices and the capacity usage rate of the added device may be leveled, so that the remaining lives of the existing devices and the added device may be leveled.

The above-described embodiment of the present disclosure is an example for description of the disclosure and is not intended to limit the scope of the disclosure to the embodiment. Those skilled in the art may implement the present disclosure in other various manners without departing from the scope of the disclosure.