Just because you don't have a floppy drive in your computer doesn't mean that the mother board doesn't have a floppy drive controller on it. Most mother boards still have the onboard floppy controller on them. Sco 6 will see the floppy drive controller, and even though there is no floppy drive on it, Sco 6 will consider the onboard floppy contoller as FD0, thus any USB floppy you connect will end up being FD1.

You then need to address the USB floppy as if it was the 2nd floppy drive, not the first.