Windows Backup and Hyper-V - My experiences (0x80042336 and failed to mount virtual hard disk)

I recently installed a virtual platform for a client and ran into a few problems when it came to backing up the virtual machines. Much reading into the subject left me rather confused as there were no real answers the problem, however I do now seem to have it working as it should be.

Setup

Server 2008 R2 running Hyper-V

1 x SBS 2011 Virtual Server

12 x Windows XP Virtual Machines

The Problem

Windows Backup was set to take a full system backup to a removable USB HDD. When the backup starts, it takes all XP machines offline into a saved state as the integration services aren't fully compatible and then tries to backup the SBS machine. After about an hour, the backup fails with error code 0x80042336 . 

If I look in the hyper-v event logs I see an error: failed to mount virtual hard disk for one of the of the VHDs.

I tried changing the job so that only the drives with the VHDs stored on them were included - no good. Started checking hot-fixes, also no good. 

The Solution

Further investigation showed that the SBS machine was waiting on the Sharepoint SP1 upgrade that I posted about previously (http://mindofdes.blogspot.co.uk/2013/10/sbs-2010-backup-issues-operation-to.html). Apparently, even though this is located inside a guest it can effect the backup. Therefore I ran this and upgraded it.

Afterwards, I checked the VSS writers on the host and saw that the hyper-v write was in an error state waiting for a previous operation to complete. I tried restarting the VSS services but this didn't help at all.

At this point, I decided to give the server a full reboot and then run the backups again - low and behold the backup burst into life and seems to be quite happy now.

Did I get lucky? Was it the SharePoint issue causing the problem? Did the first backup break the vss writer when it failed? or perhaps a combination?

Who knows - but for me the issue seems to be resolved.