Hyper-V R2: VM Backup fail with the event id 521

When you are doing a backup of VMs on role Hyper-V R2 server and event id 521 has been logged the troubleshooting is a little hard. First off, we say that the backup device is an External USB storage (HD).
The event accused:
Provider: Microsoft-Windows-Backup, EventID: 521, Channel: Application.
Now we begin with troubleshooting and here are the things that could cause this error:

1. Invalid disk signatures on the HOST and Guest
During Hyper-V backups, shadow copies of each of the VHD’s are attached to the Hyper-V server simultaneously. If any two VHD’s have the same disk signature, partition manager will “offline” the disk(s) that arrive with a conflicting (or duplicate) signature. This will cause the Hyper-V backups to fail.
- How to check for VHD signature conflicts: · Download Sector Inspector from the following location.
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=dd3ef22a-a586-4079-9489-c3ea14573fc4
· The binary is portable. Once it's installed on one machine, copy it to:
. Each of the VM's
. The Hyper-V server
· Run "secinspect -nohex > Signatures_%computername%.txt" from an elevated command prompt on:
.Each of the VM's
.The Hyper-V server
. When looking at all of the Secinspect output files, no two disks should ever have the same disk signature value. If they do, this needs to be corrected.
- What to do if you have a signature conflict: . Shut down the affected VM’s.
. Run the Disk Management MMC.
. Manually attach the all VHD’s to the Hyper-V server via Disk Management MMC.
. If any VHD’s are attached in an “offline” state, a disk signature conflict was found. To fix the conflict, right click the disk and “online” the disk manually.
. Once all VHD disks are attached in an “online” state, detach them one at a time.
. Start the VM’s.
. Verify that VM backups work.

2. Automount disabled on HOST
· Open Diskapart
· Automount Enable
· Quit diskpart
· Then try backup again, might need a reboot.

3. Hyper-V VSS writer with Windows Server Backup are corrupt.
To register the Hyper-V VSS writer with Windows Server Backup, follow these steps:
 
1.  Click Start , click Run , type regedit , and then click OK .
2.  Locate the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion
3.  Right-click CurrentVersion , point to New , and then click Key .
4.  Type WindowsServerBackup , and then press ENTER.
5.  Right-click WindowsServerBackup , point to New , and then click Key .
6.  Type Application Support , and then press ENTER.
7.  Right-click Application Support , point to New , and then click Key .
8.  Type {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE} , and then press ENTER.
9.  Right-click {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE} , point to New , and then click String Value .
10.  Type Application Identifier , and then press ENTER.
11.  Right-click Application Identifier , and then click Modify .
12.  In the Value data box, type Hyper-V , and then click OK . 
13.  On the File menu, click Exit .

And finally some useful Hyper-V related articles:

Virtualization TechCenter
http://technet.microsoft.com/en-us/virtualization/default.aspx

7 Best Practices for Physical Servers Hosting Hyper-V Roles
http://technet.microsoft.com/en-us/magazine/dd744830.aspx

Virtualization Team Blog
http://technet.microsoft.com/en-us/virtualization/default.aspx