Vintage Synth Repair

Hi All
 
Is there anyone on this reflector with experience of the GEM WK4?  I have
never worked on this beast but was asked to replace the internal battery,
which I did.  On testing it would appear that it needs to be re-programmed.
This has been tried using the original floppy disc.
 
The procedure is to insert the disc and then switch on.  A message then
appears on the display say: 'Press enter to load/ Press exit to abort'.
Pressing ENTER does nothing.  The disc is apparently rotating, the green
light showing activity is lit, waiting three minutes as instructed does
nothing.
 
The question is could the disc be corrupted or have I missed out on
something else?  Could there be a fault and if so where would this most
likely be?  
 
Fortunately I do have access to the circuit diagram and the original
operating manual.  The question is where to start?
 
Regards
Brian G3OYU
www.g3oyu.co.uk