Problem is there are a lot of things it could be.
As stated, loose or bad cable would be my first guess.
Next I'd check the diskchange mechanism on the floppy drive. See if manually sending a c:diskchange command clears the condition. Also, what does the drive do when it's completely empty? Does the icon disappear like it should, or does it stay as a DF0:
Could also be something completely stupid, though. Have you tried a virus scanner? There are a few viruses that will cause all your disks to become DF0:
. It's not uncommon to "rediscover" a 15 year old virus on Amiga...
