Error Code [91:9] Volume Shadow Copy Service (VSS) error. VSS service or writers may be in a bad state.

Article ID: HPV0004 Backup operation fails with Volume Shadow Copy Service (VSS) error.


Backup operations are failing with the following error message:

Volume Shadow Copy Service (VSS) error occurred on host [HOSTNAME]. VSS service or writers may be in a bad state. Please check vsbkp.log and Windows Event Viewer for VSS related messages, or run "vssadmin list writers" from a command prompt to check the state of the VSS writers; if the "Microsoft Hyper-V VSS Writer" reports an error, please restart the "Virtual Machine Management Service (vmms.exe)"



The error message appears due to one of the following reasons:

Cause 1:

The Volume Shadow Copy creation of one or more Virtual Machines (VM) have failed. This prevents the VMs from being backed up.

Cause 2:

VSS Writer was in an unstable state before the backup job started. This symptom can be verified by checking the status of the writer by running the command: \\\"vssadmin list writers\\\". If the VSS Writer is missing from the list, or is listed as in an unstable state, this causes backups of Hyper-V virtual machines to fail.

Cause 3:

When automatic mounting of new volumes on Windows 2008 Hyper-V host system is disabled, the online backups of (capable) guest systems will fail. Automatic fallback to an offline guest backup is not attempted.

Cause 4:

VSS Writer fails when the default path to the cluster shared volume does not match the Hyper-V Manager's path to the cluster shared volume. Mismatched paths are recorded in the following entries in the vsbkp.log:

3048 1318 08/09 09:59:23 126 CsSnapRequestor::AddVolumesToSnapshotSet() - CvClusterPrepareSharedVolumeForBackup successful for vol:[C:\\ClusterStorage\\Volume1\\] VolPathName:[C:\\ClusterStorage\\Volume1\\]

3048 1318 08/09 10:00:25 126 CsSnapRequestor::AddVolumesToSnapshotSet() - Call m_vss->AddToSnapshotSet(vol, m_shadowProvider.GetProvider(), &id) [FAILED, throwing CV exception] - Code = 0x80042306, Description = VSS_E_PROVIDER_VETO

Cuase 5:

Windows Application Event viewer displays the following VSS error message: 

Volume Shadow Copy Service error: A critical component required by the Volume Shadow Copy service is not registered.

If an error occurred during Windows setup or during installation of a Shadow Copy provider, a recent installation of a third-party hardware provider may contribute to this failure, .

Cause 6:

A volume inside one of the VMs may have run out of space, thereby preventing the Volume Shadow Copy creation.

Cause 7:

If one of the backed up VMs has information about a virtual hard disk that no longer exists on the drive, then the backup operation may fail.


Solution 1:

Identify the VM that is causing the backup failure by looking at the application, system, and Hyper-V VM events in Windows Event Viewer.

Solution 2:

Restart Hyper-V Virtual Machine Management Service to bring back the VSS writer to a stable state, which is necessary for backups. If this writer goes into an unstable state frequently, analyze the root cause from the Windows Event Viewer.

Solution 3:

Enable automatic mounting. To change the Mount Manager setting for the automount of new volumes, run one of the following commands:

  • Using the DISKPART command-line options
Command Syntax Description



automount [enable] [disable]

Reports the current setting for automatic mounting.


Enables Windows to automatically mount new basic volumes added to the system and assigns them drive letters.


Prevents Windows from automatically mounting any new basic volumes that are added to the system.


  • Using the Mountvol command
Command Syntax Description



mountvol [/n | /e]

Creates, deletes, or lists a volume mount point.


Disables automatic mounting of new basic volumes. New volumes are not mounted automatically when added to the system.


Re-enables automatic mounting of new basic volumes.


  • Using the REGISTRY Key
  • HKLM\SYSTEM\CurrentControlSet\Services\MountMgr\NoAutoMount

    0 - enables automatic mounting


Solution 4:

From the Hyper-V Manager console, rename the path to the VHD. For example, if the path to the VHD file is: \""file:///C://ClusterStorage//volume1/">C:\\ClusterStorage\\volume1\",

Rename it to  \""file:///C://ClusterStorage//Volume1/">C:\\ClusterStorage\\Volume1\" to match the case. Note that the only difference is the case of the letter 'V'.

Solution 5:

  1. Identify and temporarily uninstall any third-party VSS Hardware provider (Example: NetApp SnapDrive).
  2. If the backup succeeds, re-install the software.

Solution 6:

  1. Make sure that the volumes in the Guest operating systems have a minimum of 500 MB free space.
  2. Run the backup operation again.

Solution 7:

  1. Identify the virtual hard disk that no longer exists, and remove its information from the Virtual Machine settings
  2. Run the backup operation again.