I'm revisiting this as I had the same problem again on a different machine, and think I may have worked out what is happening. The situation was exactly the same - I was migrating to a new SME10 on new hardware and using the server manager backup/restore function. I'd taken the backup, plugged the backup drive in to the new machine, configured workstation backup, and then gone to restore from workstation. I got the same error screen again (I've attached it to this post again). Plugging the backup drive into the old server and going to restore from workstation and the disk was mounted fine.
So what is going on? My clue was that the backup disk had no volume label, and I could mount it fine manually. I changed the volume label to backup2 on a Mint box, and then plugged the backup drive into the SME 10 server. Reconfigured workstation backup, then went to restore from workstation, and it all worked fine. Now, I've not raised a bug yet, as I want to see if I can replicate it again, and also I want to see if it is just a problem when you are migrating from 9 to 10. If the team would rather I raised a bug now, then please let me know, and I will do so asap. Thanks.