The backup did not complete because of an error writing to the backup location B:\. The error is: The backup location cannot be found or is not valid. Review your backup settings and check the backup location. (0x81000006).
Windows 7 logs this event when a scheduled backup didn't start, because the backup destination (B:\ in this case) was unavailable.
This could be because a removable USB device was unplugged, a network location is unavailable, etc.
0x81000006: The backup location cannot be found.