Agentless backup of Hyper-V virtual machine(s) produces the following information message:
Changed Block Tracking (CBT) is not used because the differencing virtual hard disks of this machine contain unmerged changes.
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.
3. Another backup is running.
1. Verify the backup schedule. If you back up virtual machines and hosts in one plan, please create separate backup plans for virtual and physical machines and move start time of VMs' backups so that they do not overlap with the hosts' backups.
2. Ensure that there are no other backup tasks running on hosts or virtual machines at the same time. Check plan schedule of Acronis Agent, Windows backup and other backup tools if you have any.
E.g. the following message in Windows Event logs indicates start of Windows backup:
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.