A medium storage device writes the data on a medium by a write element and executes the diagnosis of a write system path without dropping the performance of the device. For a write command from a host, write data is stored in a data memory, a response is returned to the host, then the data is written back from the data memory to a storage medium. During this write back, partial data, including the final portion of the write data, is read from the storage medium, and the data is compared. Therefore the write system path can be diagnosed without affecting the host, and since partial data including the final portion is the target, diagnosis in a short time can be implemented, and a drop in performance can be minimized.