A recent upgrade to LR5.6 no longer allows me to locate pictures on external drives with Mac OS10.8.5.
Why not? Exactly what steps have you tried? What happens when you try?
External Drive 1 and Drive 2 are not listed in any menu allowing me to point the way to the file.
Well, it still would help if you stated "exactly what steps" you are trying, as I asked above. That would lend some clarity to the discussion, as I can think of a couple of reasons you might want to do this and a couple of places where you might (or might not) be looking. I think to move forward, we need these details.
However, have you tried this: Adobe Lightroom - Find moved or missing files and folders