I have already given up on having Parallels directly support accessing a real USB floppy drive the way it can access the real physical optical drive... But now I'm having the problem that Parallels says that 720 KB floppy disk images are invalid. I have tried creating them with OS X's Disk Utility from a USB floppy drive, and WinImage on a true-blue Windows machine with an internal floppy drive. 1.4 MB images (as .dmg renamed to .fdd from OS X, and as raw .vfd from WinImage,) work just fine. But 720 KB images don't. For some old programs, I can just copy the files from the floppies directly onto the mounted .hdd image, and install from a directory on the virtual machine. For others, I can just mount the 720 KB image and a blank 1.4 MB image in OS X, copy the files from one to the other, then use the functioning 1.4 MB image in the virtual machine. But I also have a couple of boot disks that I want to mount, and neither method works.