The slot the original SSD was in. Same form factor and everything (except the physical shield, which doesn't seem to make a difference (so far! ;-). Apple uses Samsung SSDs for their drives in that model (or, at least, that's what they used originally -- maybe they've changed since then), but the form factor is the same. So: pop out the old, pop in the new. No unscrewing of security screws or anything =)
Okay, editing because maybe that's not what you are asking. To be clear, I *replaced* the original SSD. AFAIK Stating as fact: there is no slot for an additional SSD. Which, yes, is a bummer. But this thing takes up a fraction of the space the cheese graters did and so far out-performs them handily for what I do. And external drives have proved to be no problem so far -- I have three externals, whereas I had four internal in the old one, and I've noticed no detriment in performance so far ...
Obviously, YMMV =)
Last edited by six_of_one; 08/22/15 03:36 AM.