Thankyou for your post. That website was very helpful (it is now bookmarked :P), but I was still unable to resolve the issue. I copied my image to a floppy disk and stuck it in an old pentium 2 pc (that has proper, IDE floppy drive), and it works fine. I'm convinced now it's something to do with the way USB floppy emulation is handled in USB devices, and the drive id in the dl register.