Symptom
For specific virtual machines in a subclient, an incremental or differential backup automatically converts into a full backup. The following event message is displayed:
Changed block tracking verification failed for disk [%s] on virtual machine [%s]; performing a full backup of the disk. The virtual machine may have been vMotioned since the last backup. If this issue persists it may be necessary to reset changed block tracking on the virtual machine.
Cause
The first backup for the virtual machine becomes a full backup after you migrate the virtual machine to a new datastore.
This is because Changed Block Tracking (CBT) is reset after a storage vMotion operation. For more information, refer to Change Block Tracking is reset after a storage vMotion operation in vSphere 5.x (2048201).
Resolution
All subsequent backups in the backup cycle are performed according to the schedule for the migrated virtual machines.
To reset CBT, see Reset Changed Block Tracking (CBT) for VMware backups.