Have you tried the hard reset..

Switch keyboard on whilst turning the tempo dial round and follow on screen propmts.

Secondly do you have the original operating system disks supplied with the keyboard as you can try loading them then updating to the last version.

failing that if the disk drive is not reading it could be the microchip on the cpu board that controls the disk drive. these have been known to fail and are not a hard repait providing you have an engineer to do it.

i have a copy of the schematics on disk somewhere for wk4 and also the o.s discs however these can be downloaded from the gem media soft website.

come back to me if you have no joy