Open preferences > Options and make sure that the Aperture 1.5TB drive is NOT in the list of excluded drives
Should do it for you
BTW. No need to post this question 3 times, unless you have an itchy trigger finger that is.
I ran out of room on my 500g iMac-so I plan to move my aperture library to a 1.5T external drive. I have a second 1.5 T drive that I want to use to back up the library drive AND the imac using Time machine. How do I connect everything and set up time machine to back up the mac and ex. drive?