The identification information about unnecessary files is registered to generate an unnecessary-file database. The identification information about files stored on a plurality of storage devices connected to a host computer is acquired to judge with the unnecessary-file database whether the identification information about files conforms to unnecessary-file conditions. Files nonconforming to the unnecessary-file conditions are checked to determine whether they are already migrated to a different storage device. If all the files stored on a storage device conform to the unnecessary-file conditions or are already migrated to a different storage device, the identification information about the former storage device is displayed to indicate that the storage device can be removed.