Is it possible to use a HD floppy disk drive in an Amiga?
Yes, it is.
You only have to modify the cable, connecting the floppy and the Amiga a bit.
You can find a how-to in the Aminet (just google for it).
And this is, how it should look like:
And I used a "normal" Diskdrive for it. Just went to a store and said "Yo, I need a Floppydrive" and it worked.