Yes, you can use a internal drive for making one external, but you have to use a special interface that i saw many time ago. On aminet i think i saw once a schematic to do that.
On the other hand, no, you can't use a pc disk on an amiga directly, they are not compatible. There was an adaptor made by individual computers called "kylwalda", that could allow to use any standard pc drive to an amiga, and it even worked with the games and ndos disks. This interface hasn't been produced since years, so now it's impossible to attach a pc drive without hacking and messing with cables, etc. It could be done, but you will need a bit of soldering skill to get done. Search also on aminet for this hack.
Some old pc diskdrives as the Teac, could be setted by jumpers to act as an amiga floppy, but these are nowadays hard to find.
Good Luck and hope this helps.
