Sierra won't stop indexing my backup drive

Has anyone else had this problem with MacOS Sierra?

Some of my SuperDuper cloned back-ups for my two Apple computers will not allow me to prevent indexing them after I attempt to drag the backup drive icons into the Spotlight Privacy panel.

I get an error message that says? Privacy List Error (null) The item couldn't be added or removed because of an unknown error.

Never had this problem before until I switched to Sierra.