Just telling you guys, that 2011-12 MacBook Pros does NOT support Sata 3 interface in the optical bay.
Apple tells you they do, also sys info tells you too, however they don't. If you use them, there will be I/O problems. Sata 2 interface can be used without problems in the DVD bay. The main bay, can be used for both Sata 3 and 2.
13' laptops CAN support Sata 3 in the optibay too, but 15 and 17' models cannot.
I have a late 17' model, and I am having the same issue. I want 2TB in the optibay, but seems there aren't any sata 2 interfaced drives out there.
Also, I have to mention that there WAS no point of drive makers to use Sata 3 on HDD's, since a HDD is only capable of maximum 180MB~ /s reading speed, which Sata 2 could easily handle. (Sata 2 has a cap of around at 330MB/s and Sata 3 is around 660MB/s, thats why apple uses PCIE SSD's in their laptops now, to be able to achieve 900MB-1GB/s speeds.