Vista - Complete PC Backup error 80070079

I am running Windows Vista Ultimate on an HP Blackbird. I purchased a 500 GB
external HDD from Cavalry about 6 months ago (pre SP1) and had no problems
doing either complete PC backup or backup all files. Now I can still backup
all files, but cannot do a complete PC backup and get the above error code
(semaphore time out). The cable is fine, and I get the error with either
SATA or USB.

Cavalry has no device driver for the disk and relies on Vista, and like I
said, it used to work fine. A Google search shows some others have had the
problem, but no solution.

I burned a recovery DVD, but would like the convenience of using Complete PC
Backup just in case.

Any help would be greatly appreciated!