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Hyper-V CBT error CBTDriverError:uvbCbtSnapshotCreationStarted


CBT drive on a Hyper-V cluster can get out of sync.  This can cause CBT issues with getting changed blocks resulting in incremental backups that are the same size of a full backup.


----- Error Messages -----
CBTDriverError:uvbCbtSnapshotCreationStarted:: FFFFFFFF80070002
Due to previous error, Unitrends agent is not able to use the Hyper-V CBT filter driver during backup job for VM {A4C9989D-86C9-4769-81FE-E8765E5B6691}: all files will be backed up as full not change blocks.
----- End Error Messages -----


Open the following file in notepad and review the Sync Status line on each Hyper-V node in the cluster.


You will see if the drive is InSync or NoSync or OutSync like the following.


If the driver is out of sync you will want to stop the CBT (UnitrendSVC) service on each Hyper-V node.

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Once the UnitrendSVC service is stopped on each node, start the UnitrendSVC on each node to complete the sync CBT.  

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Lastly, take another Hyper-V full backup which, once completed, should remedy the incremental backup size issues moving forward.

If the issue persists, rip and replace of the driver across all Hyper-V nodes before reinstallation may be necessary.

The steps for removing/reinstalling the agent and CBT driver when having CBT issues are as follows:

  1. Copy C:\PCBP\keystore somewhere for safe keeping.
  2. Stop the BP_Agent service
  3. Uninstall the current Unitrends agent
  4. Verify that C:\unicbt does not exist, if it does exist run as admin-> C:\PCBP\Win2012.dir\<CBT Driver Directory>\uninstall.bat  (IE: C:\PCBP\Win2012.dir\07-28-2020,\uninstall.bat
  5. If following the above mentioned has been ran and the C:\unicbt directory still exists, you may need to reboot the host and re-run the uninstall script as mentioned above.
  6. Once the C:\unicbt directory is gone, remove the PCBP directory entirely (backup keystore! step #1)
  7. For Hyper-V clusters, perform the above mentioned on all nodes before continuing.
  8. Run the latest agent installer
  9. Copy keystore contents back to C:\PCBP\keystore to ensure SAP continues to work.



CBT drive loses sync with the other cluster nodes which requires a reset to get the driver back in sync.

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