A flexible magnetic disk drive is disclosed which is linked to a computer via a universal serial bus interface having firmware held on a read-only memory. In order to facilitate the upgrading of the firmware, an electrically erasable, programmable ROM is employed for firmware storage. Each new firmware version is issued in the form of a flexible magnetic disk which may be loaded in the disk drive just like an ordinary data disk, only with the disk drive disconnected from the computer as far as data transmission is concerned. The EEPROM is preprogrammed to identify the loaded firmware disk, erase the old firmware version on the ROM, and write the new version thereon. In another embodiment a self-testing program disk is employed in place of the firmware disk, for performing a set of tests on the disk drive including the interface. The tests are conducted automatically as the self-testing program disk is loaded in the disk drive, again with the disk drive disconnected from the computer.