Abstract: In an apparatus and method for ensuring data in an external storage system, the system includes an external storage controller for transferring read data from at least one external storage device, such as a magnetic disk drive, to a host device and for transferring write data from the host device to the external storage controller. The external storage controller adds to the write data from the host device check data including distinctive information on a position of the external storage device where the write data is written, transfers and writes the write data in the external storage device. Further, the external storage controller checks the check data included in the read data from the external storage device when the data is transferred to the host device by comparing the check data with a control check data computed and stored in the external storage controller.