I am trying to fix an external Disk /// drive that gives a "Format error #21. Internal program error." Message when I try to format a diskette. When I format the diskette using the internal disk drive, the disk completes formatting with no errors. I've looked through my Apple /// manuals and did a Google search, but cannot find what this error means. I've cleaned the head on this disk drive, have cleaned the head mechanism rails with isopropyl alcohol, and have replaced the ICs on the Disk /// Analog Card from a known working Disk II drive. Appreciate any help/information on this error and suggested fixes.