SUMMARY

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.

ISSUE

----- 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 -----

RESOLUTION

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

C:\unicbt\config\in.xml
You will see if the drive is InSync or NoSync or OutSync like the following.
<CBT>
<VERSION>2</VERSION>
<SYNC_STATUS>NoSync</SYNC_STATUS>
If the driver is out of sync you will want to stop the CBT (UnitrendSVC) service on each Hyper-V node.

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

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

CAUSE

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