@Doomy
Thats not the source of his problem, even if HD disks will have a shorter life-span in a DD drive, they will still work for quite a while...
@Deltawolf
If you have Crossdos activated, and you insert a PC-formatted disk in the drive, it wont turn up as DF0:, it will probably turn up as PC0:, in which case you have to format the disk as an Amiga disk instead. Turn off Crossdos and try again.
Or perhaps you are using a Catweasel? Drives connected to a Catweasel will get different unit names than if the drives were connected directly to the Amiga.