Agentless backup of Hyper-V virtual machine(s) produces the following warning:
Changed Block Tracking (CBT) is not used because there are disk files that do not belong to any disk snapshot.
Possible causes are:
1. Issues with the checkpoint tree of the affected virtual machine.
2. The situation when the backup starts at the moment when some checkpoint was already deleted, but the corresponding avhdx-file is still merging with the vhdx-file.
1. First, make sure there are no running tasks related to a checkpoint deletion. If any such tasks are in progress during backup - start the backup later or distribute these events in time in order to avoid such warning in future.
2. If there are no running tasks related to checkpoint deletion, then delete all the checkpoints you do not need and reboot the virtual machine that produces warnings.
3. If the above solution does not help, merge avhdx-files manually (see this page for instructions) and reboot the virtual machine before running the backup.