My external "working" drive for Lightroom CC filled up, so I added a second external named "working-2" to my catalog. Lightroom is storing photos on the new drive but I am not able to back it up. I get an error message that the working drive or the backup drive are full. Neither is. What do I need to do to back up the new drive?
I'm not sure what that error message means (could you quote the EXACT word-for-word error message and state when it appears?), but to make backups, you simply copy the photos from one drive to another drive.
Can you post the screenshot for the error message that you are getting.