PROBLEM

In a Hyper-V environment, backups of VM(s) on specific Hyper-V Host(s) are failing with error:

A problem with Windows Shadow Copy was detected

VSS Error Code: 790

CAUSE

Microsoft Windows was unable to complete a successful Shadow Copy (VSS) at host level.

SOLUTION

1. Ensure that there is a minimum of 10% free disk space on each of the source drives (including CSVs) where your VHDX files are located.
   
   

2. Check the Windows 'System' event log for any disk issues, normally coming from source 'DISK' or 'NTFS'. Disk errors can and normally will cause VSS failures, thus should be looked into before proceeding with the backup.
   
   

3. Run the command 'vssadmin list providers' on the host machine and check if there are any 3rd-party VSS Providers listed aside from the default Microsoft providers. These can sometimes be left behind by other applications and cause such VSS 790 errors.  If this is the case, the 3rd party providers should be be uninstalled/removed
   
   

4. Check if there is any other backup software running at the same time, this can cause VSS conflicts resulting in the above error.  If so, schedules should be adjusted so that the two products do not run concurrently.
   
   

5. Check for event's with the source volsnap or VSS and Event ID 8193 with a description that includes:

   "Cannot find anymore diff area candidates for volume "
   
   
   
   In order to resolve this issue, you'll need to manually assign the shadow copy location for each of your source drives. For example if you have 2 source drives on your host, C: and F: you will need to run the following commands for each of your drives.
   
   Change the drive letter accordingly:

   vssadmin add shadowstorage /For=C: /On=C: /MaxSize=UNBOUNDED
   vssadmin resize shadowstorage /For=C: /On=C: /MaxSize=UNBOUNDED