If the drive works over firewire, there's no reason it shouldn't work over USB. There are three possibilities on why it's not working. either the usb cable is bad, the usb port on the drive is bad, or the usb port on the computer is bad. There is one outlying possibility, that being that the computer isn't recognizing the drive over USB because it remembers it over firewire, but a reboot would wipe that out. One thing you could test is to plug it in and look in the system profiler under the USB section. If the drive is connecting properly, it should show up there. If it is there, you might try disk utility to see if it's also there. I forget, but you may be able to force the drive to mount from there it's listed.