I haven't used Ghost 2003 for a couple of years now. I do remember that using USB drives with Ghost can be problematic. It's not that it can't be done. It can. But there are some potential hangups.
I remember issues with systems that contain both USB 1.1 and USB 2.0 devices. Using an internally-attached hard drive, or using a networked shared drive (even a shared USB drive attached to ANOTHER PC across the network) avoided the Ghost USB driver issues.