adding disk target to existing windows backup
If you need to add a new external disk to an already configured Windows Backup rotation, run the following commands from an elevated command prompt after connecting the new disk.
wbadmin get disks WBADMIN ENABLE BACKUP -addtarget:{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
Use the results from the first command to ID you new disk(s) and enter the respective disk ID into the second command to add it to the rotation.