Same problem here, just bought a HP690 Mediasmart server, after a nightmare setup with the network, everything appeared to be fine until I tried to restore the first backup I made to test it.
Stops at 81 percent, and says "cannot mount the backup"...that's it. Can't even access the files through RDP.
Have all the latest patches and upgrades.
Doing many searches I can see this is HUGE problem with WHS, but there is not one solution, people reported success from updates, but I have all the updates.
So frustrating.