perhaps the issue is UBCD doesn't like USB connected storage?? does it load USB drivers?? just because the BIOS knows ur hdd is plugged in doesn't mean the UBCD will be able to access it. If it doesn't have software to drive the USB device and use it, then this may be whats happening. I can't remember using UBCD for USB devices, when I was working I had a testbed that I'd hook everything upto.
I'm just stuck on why you NEED UBCD if something like Seatools can still do the job? is Seatools slower?? sry, bear with me.