I have a 2012 server running Hyper-V with 4 VM servers running 2012. Backup of all of them is done from within each server using Windows Server Backup.
I have a removable drive system that uses 1 TB cartridges. On each cartridge I created a VHDX file for each of the 4 VMs to attach to each and use as a backup drive in WSB. The cartridges are swapped periodically. Since both drives can't be online at the same time, to set this up, with cartridge #1 in place I attached the VHDX in Disk Management on each server. Then in WSB I added the first attached drive. Then I swapped in cartridge #2, attached the VHDX in Disk Management and using wbadmin at the command prompt, I added the drive from that cartridge.
This was working fine until recently when I started having troubles with the backup of 2 of the VMs. On cartridge #1 all backups are working fine. On cartridge #2 two of the backups work fine but two do not. After trying to resolve the problem without success, I finally decided to delete the VHDX file for those 2 servers on cartridge #2 and create new ones. But when I tried to add them via wbadmin after the drive is formatted and labeled by wbadmin there is another line displayed that says "The system cannot find the path specified". Because of this WSB does not see the second drive.
I've used this same setup on other Hyper-V setups and never ran into this. I've run out of things to try or look at. I hope someone here has an idea of why I would get that message at the end of the process: wbadmin enable backup -addtarget:{disk identifier number}.
Jonathan