Here is the error code: backup error 0x807800C5

This was in the log file:
Backup of volume \\?\GLOBALROOT\Device\HarddiskVolume1\ has failed. The mounted backup volume is inaccessible. Please retry the operation.

I found this suggestion and did it: http://www.tenforums.com/performance-maintenance/18417-system-image-backup-807800c5-error.html

But it still didn't fix it.