This is what worked in my case:
Go to the properties of the external hard driveUncheck the "Compress this drive to save space"Depending on the size of the drive and the files on it, it may takesome timeRestart and attempt another full back up
In my case it worked when all previous attempts failed with thatcryptic error code: 0xC00000EA. Apparently, Vista has a problem backingup data to a compressed drive.
Error: 0xC00000EA - An error occurred. The following information might help you resolve the error:
If an MM error is returned which is not defined in the standard FsRtl filter, it is converted to one of the following errors which is guaranteed to be in the filter. In this case information is lose, however, the filter correctly handles the exception. This error occurred on computers running eTrust antivirus software. Excluding zip files from being scanned or disabling eTrust solves the problem. Contact CA, the antivirus vendor, for an updated version.