@aghering the [NOBAK] option only excludes the transfer process, it will still try to make a snapshot of the disks. There are many possible causes for your UNIMPLEMENTED IN SM BACKEND issue, the most likely causes are probably storage related. If it is not a critical VM, you can try to disconnect the second storage disk and try again. If that doesn't work you might have to forget the SR and try (you can reattach it again, just need to remember your sr-uuid.
Also, your /var/log/SMlog might reveal something.