It is not a good idea to put fast SSDs into the optibay. System tries to negotiate at 6 Gbit, but it fails; disk is recognized but it couldn't even be formatted. I've waisted a lot of time to find a reason. After swapping SSD with original mac's HD both work properly, one at 6, another at 3 Gbit
I'd to disassemble my MBP two times because my SSD did not work properly in optibay. While doing it a second time, I omitted steps 6 and 8. It is really not necessary, but step 6 is really scary.