W2008 R2 adding a source drive fails "The Filename, directory name, or volume label syntax is incorrect"

I've a scheduled backup running to multiple removable disks. Some are offsite. Windows server 2008 R2.

I have added a new volume to the server that needs to be backed up to the same set of removable disks. 

However from the GUI when I try to modify the backup to include the new source volume, I get the "The Filename, directory name, or volume label syntax is incorrect".

Have I got to take all my offiste disks onsite, delete the entire backup job (and loose all my backup history) and recreate for the new source set, adding one target volume at a time? OUCH!