I wish to provide a solution that took me months to find for my Live VM backup on a Server 2008 R2 running 2 VM/s (SBS 2011 & Server 2008 R2/SQL Server 2008R2).
If you recieve an error EVENT ID 521 with the included error code (2155348129) in the event details in your hyper-v host system - and you have exhausted all other possibilities - then have a look at this solution. It may be the hidden gem that will let you sleep at night.
I had been unable to find a solution for performing a FULL BACKUP (Using Microsoft Windows Backup) with my VM's running, and all persons, help tickets, advice, etc., just kept me running in cricles around the same solutions that everyone else seems to have tried - some with success and some without.
Most seemed to come down to the Sharepoint psconfig.exe command needing to be run. Not my problem nor was any thing else I could find. Some including setting your system reserved as an "ACTIVE" partition - DON'T DO THAT!!!!! You will likely end up with a non-bootable system!
Others wanted you to create the VSSACCESSGROUP in your registry and then alter your DIAG key permissions (although all this does is help you clear out some Microsoft "benign" critical errors that may be safely ignored. And the list goes on. None of which worked in my situation to resolve my failed backups.
Well, yesterday, in searching for information on an unrelated issue - I stumbled across this article (mentioned below) that was my salvation.
Finally a solution for my failed backups problem!!!
No where had I seen or heard of this issue.
My volume structure was a little different but the solution was the same.
(NOTE: I peformed the following on the PHYSICAL VOLUME as well as the VM VOLUMES!)
So the credit goes to Dustin Puryear @ http://www.puryear-it.com/
And here is the direct link to the page to the solution.
In Windows 2008+, you will notice that the Windows installation creates a 100MB partition (the System Reserved Partition [SRP]) that is not assigned a drive letter but is mounted to C:\Windows\SRPPartition. Here's the problem, when you are doing a full bare metal recovery backup, which includes the system state, just the system state, or trying to back up everything, WSB will use VSS to backup the partitions, including SRP. But 100 MB is too small to allow for this! Not a very smart design choice. You can confirm that this is the problem by removing the bare metal and system state backup options, and only backing up your C:. That will work. But add system state, and it fails.
The solution is to have VSS use the C: or another drive when creating the snapshot of the SRP. This way the limited 100 MB is no longer an issue.
1. Start Disk Management.
2. Click on the SRP.
3. Right-click the SRP and choose Properties.
4. Click on Shadow Copies.
5. Select the SRP.
6. Click on Settings.
7. Notice that the default is for VSS to use the SRP itself for the snapshot.
8. Choose the C: or another drive with ample space, set the Maximum size to No Limit, and click Ok.
9. Click Ok to exit the System Reserved Properties window.
10. Now run WSB and backup the System State. It should work just fine!