CBT Driver error occuring when backing up a VM with a disk that was previously attached to a different VM


This article explains how to resolve a Hyper-V CBT error that can occur when backing up a VM to which you have attached a disk that previously attached to another VM protected by your Unitrends appliance.



If you detach a disk from a VM protected by a Unitrends appliance and then attach this disk to a different VM, you might see the following error when running a full back up for the different VM to which you have attached the disk:  "Due to a previous error, the Unitrends agent is not able to use the Hyper-V CBT driver during a backup for the the VM.  All files will be backed up as full, not changed, blocks."  This error occurs because the CBT driver tracks changes by referencing the disk in relation to the original VM.  As a result, the driver detects a conflict when the disk is attached to a different VM and does not provide changed blocks.


You can resolve this problem by performing one of the following:
  • Rename the disk that was originally attached to a different VM or move it to a different directory.  This is the recommended resolution because it does not impact backups for other VMs residing on the same host.
  • If you prefer not to rename or move the disk, you can uninstall the Hyper-V CBT driver on the host and reinstall it.  However, this resets tracking information for all VMs residing on the Hyper-V host, so the next backup for every VM on the host will be automatically promoted to a full.  For instructions on installing/uninstalling the CBT driver, see . . .


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