Eject destination on completion is "dim"
 

I back up to a separate internal drive inside a Mac Pro. When I attempt to set the Eject destination on completion feature that menu option is dim.

How do I get it set to eject this internal drive when finished?

At present, internal drives can't be unmounted (we're using Finder's indication of 'eject-worthiness' when enabling the option), but I've reported that as a bug... hopefully the next version will allow it.