SymptomsWhen trying to perform backup of a virtual machine which has HDD file outside of .pvm bundle it fails with the similar error:
~# prlctl backup VM-with-external-HDD
Backing up the VM VM-with-external-HDD
Unable to synchronize the virtual machine's file system. The backup process will continue without synchronization. To avoid this problem in the future, make sure that Parallels Tools are properly installed in the virtual machine.
A full backup is being created for this virtual machine. Though an incremental backup is created by default, it cannot be made because the virtual machine has no full backups.
backup hdd.0 progress: 100%
Failed to backed up the VM: The backup command for the "VM-with-external-HDD" virtual machine failed with code 7 ("Open error"). Try to back up the virtual machine again. If the problem persists, contact the Parallels support team for assistance.
CauseThe design of Parallels Server backup tool does not allow to backup .hdd files which lay outside of virtual machine .pvm bundle, to avoid issues during restoration procedure.
In order to get backup of .hdd file it is necessary to store it inside /vz/vmprivate/VMNAME.pvm/ folder.