How do get my USB drive back as a Time Machine backup option?

I erased my USB Disk called OLD HDD that I was using for TM backup. I reformatted it as an encrypted MAC Extended (Journaled, Encrypted). When I tried to have TM use it as a backup drive, I accident'y pressed the "-" in the Exclude these items from backups window and it disappeared. "oops!" When I click "+" in an attempt to add it back, the Old HDD backup drive shows but the only option is to Exclude. How do I get the Old HDD back as a backup drive? I am using a MBP mid-2012 with SSD and 16gig memory upgrades. Pic added.