Hyper-V VHD files inaccessable after reinstallation from Hyper-V host

Today I had a funny session with my coworker Jens Nilsson. Last week we encountert an issue with his Hyper-V cluster and we needed reinstall one of his hosts.

After we reinstalled the host and tried to import some of his no clustered VMs, stored on a local drive, we encounter follwing issue.

Log Name:      Microsoft-Windows-Hyper-V-VMMS-Admin
Source:        Microsoft-Windows-Hyper-V-VMMS
Date:          9/19/2014 4:11:06 AM
Event ID:      12620
Task Category: None
Level:         Error
Keywords:

User: SYSTEM
Computer: HV02.test.local
Description:
”: Cannot get information for attachment ‘D:\virtualmachines\VHD\CLY01.vhdx’. (Virtual machine ID )

Microsoft-Windows-Hyper-V-VMMS-Admin
HV02.test.local

D:\virtualmachines\VHD\CLY01.vhdx
%%2147944320
0x80070780

So at first we thought it was an security issue and we checked those. They were fine and no services were blocking the access. During our discussion, Jens told me, he was using Windows Server 2012 R2 data deduplication on that drive with the old installation.

Ok that was the information I needed. In our case that meas, the file was still on the disk but when you tried to open it, Windows was missing the index to reassemble the blocks belonging to the file.

Now comes the issue, how can we fix that. Our first and for us both studid idea. Lets install data depluction again. Maybe he will be able to find his index again. Aehm … you know what happens with stupid ideas? They often work, in that moment too.

Next we wanted to find out why, the only possible solution was, the index for dedup is stored in the system volume information of the drive where it is configured. Which means, when you backup a drive where you use dedup, do not forget to save the system volume information too.