Hiya,
Another thing i just remembered, The floppy drive in the W7/W5 series can only read double density floppys. If you are saving midi file on your computer onto a High density disc,the chances are that the W5 will not read it.
The W5/7 can read double density floppys that are formatted on a pc.
Try this to be on the safe side,----Format a double density floppy on the W5,put it into the pc,save a midi file onto it in Format 0,put it into the W5 and it should load the midi file.
Let me know if it works!
Also the W series synths were fitted with a problematic disc drive but if this is the case the error message would read "Disc not ready".
I dont think this is the problem in your case though.
Hope this helps.
Mick.