Since their release, people have been asking whether the 2011 MacBook Pros can use a 6Gb/s SATA drive at full speed in the optical bay, via an adapter such as the OWC Data Doubler. The short answer is: No, you can’t.
Unfortunately, the optical drive bay only runs at SATA 2.0 speeds (3Gb/s). There doesn’t seem to be any firmware setting to enable the faster SATA 3.0 protocol. This is a factory limitation, and we don’t expect any updates from Apple, since they don’t even seem to be officially supporting 6Gb/s right now, anyway.
To reiterate, this is a hardware limitation of the MacBook Pro, not the fault of the 6Gb/s-capable drives themselves. However, it does change the way you may want to look at upgrading your machine.
For best performance, you’ll want to put a 6Gb/s SSD (such as the OWC Mercury EXTREME Pro 6G SSD) in the original drive bay, and use a Data Doubler to install your factory hard drive into the optical drive bay. You’ll get the SATA 3.0 speed boost from the SSD, but still have the high-capacity storage your standard hard drive provides.
If, however, you’d rather not put your hard drive in the optical bay but would still like the speed boost an SSD provides, then we suggest getting one of our original Mercury EXTREME Pro SSDs and pairing it with a Data Doubler, instead.