USB floppy disk drives are only any good at reading and writing 1.44mb disks. If you need a 720k disk for something like an Amiga/Atari ST they don't work IME (some people have claimed otherwise but I've never had any luck).

I keep an old Core Quad machine with a real floppy disk controller on it just for this reason. Windows XP / XP 64 and Linux are the only things that write to these drives properly (IIRC Windows versions after Vista don't control the floppy controller directly). I do have better options now for transferring files to these old machines.