1. You have created a backup of a virtual machine with large virtual disks (disk size more than 300Gb),
2. When you mount a backup using Run as VM option, operation finishes with the following error:
Failed to prepare operations. Error code: 7 'Disk is not found. Ensure that all the hard disks are installed in the machine.'
Issue in the product.
The issue has been fixed in Update 5 (Build 16180)