My guess is the larger one isn’t using a Proxy which can access the larger VM disks on the Volume and is failing back to nbd. Within the job history (double-clicking the job), is the larger VM indeed using the same proxy? Select the smaller VM and see it’s using the Proxy with direct access to this Volume. Then, select the larger VM and verify it’s using the same Proxy. Can you take a screenshot and share it, blurring out any pertinent info?

Check in the logs here - C:\ProgramData\Veeam\Backup\Jobname

This will have more details on the issue hopefully that you can post here.  Unsure why one VM would back up using SAN then the other NBD.  If the configuration is still the same it should work.

Another thought I had @spider32 - is the larger VM by chance connecting its CD drive to some ISO with ISO storage on another datastore? Your large VM will show as having a 2nd datstore connection to the datastore housing ISO storage. And, your Proxy doesn’t have access to that datastore. 

Yes, both of them are actually using the same proxies and these proxies have direct access to the volume. I will attach a screenshot once able.

I haven’t looked at the logs yet, just really curious about the behavior where the only change was the volume. Will let you know if I find something conclusive in the log.

Thanks again for your feedback!

I should have looked if there were any stale snapshots. The VM that is being backed up using NBD had a stale Veeam snapshot causing it to be backed up using NBD. The stale snapshot got created around the same time the VMs were moved to the Large File volume.

Thanks for all your suggestions!

Glad to hear you figured it out.

Awesome. Glad you were able to see what happened.

Interesting...I didn’t know that was a thing it would do.  Thanks for posting the root cause.

Thanks for posting what was causing it.   I just Veeam Zip things now or use storage snaps and never run into this anymore.