To start with, USB hard drives are prone to give problems when trying to backup large files on a network. The problems can be reduced, to some degree. Yet the obvious solution is to use another kind of interface for network backup.
One problem is documented in the MS KB. And besides that, the USB interface can fail for hardware reasons.
First thing I would do is pull the drive out of tghe USB thing. I did that with my 80GB Maxtor and found the drive worked perfectly but the USB interface had bit the dust from my mistreatment. I am very tough on poorly made stuff, I break anything that is not idiot-proof.