I found it when I was on the external drive but once I clicked on it, I got an error message and then when I tried again, it just disappeared into thin air. Now I cannot get back to that library on either computer for some reason. It should just be click on Go in Finder and the Option. No so anymore.
I am trying to use SuperDuper now and am at a standstill. It says after I partition the ext. drive (already done) and name the partition, I am supposed to click the "apply" button but I don't see one.
Anyway, still happy I have my old MBP up and running!
Have you looked for it in your external Mavericks clone?
More: Since it's apparently irreplaceable, I suggest that if you find it, you do a control-click > Compress before you install it, so you'll have it backed up.