Time Machine excludes the disk I want to back up.

Time Machine got glitchy six months ago so I started using Carbon Copy Cloner, which does not do separate backups each day/hour. Now I want to go back to TM, but when I set up TM, the disk I want to back up is "Excluded" and I can't delete it from the list. Running 15.01 on a 3 year old 27" iMac we 48GB of RAM. . "Files" disk is a separate 10TB LaCie drive from internal OS SSD. I want to back up "Files", which has about 3TB on it. The OWC drive I am trying to use as a TM is 6TB. I have done restarts, renamed both drives, restarted again, partitioned the TM drive.....

Again, Time Machine forces me to "exclude" the Files drive that I want to copy.