I have an early-2011 MacBook Pro running OS X 10.10.2 (also Windows 7 Professional on a Boot Camp partition), and recently upgraded my hard drive to a 500GB SSD (Crucial MX100). The speed is amazing, but I don't want to load too much data onto the drive since TRIM cannot be enabled in Yosemite without disabling kext signatures (I'm utilizing the disk garbage collection and do not wish to enable TRIM; the more disk space available, the better the garbage collection firmware works on the SSD).
That being said, I am considering swapping out my SuperDrive for a second hard drive – keeping the OS and programs installed and running from the SSD, and saving documents and files on the HDD. However, I am aware that in some 2011 MacBook Pros, the optical bay can use either a SATA II or SATA III connection. I checked in System Profiler, and it shows that the optical drive's link speed at 6 gigabits, with negotiated link speed at 1.5 gigabits; therefore I'm guessing it's a SATA III capable connection that's simply throttled to the speed of the optical drive. However, reading here would suggest that the 17-inch early-2011 MacBook Pro may not support a 6.0 gigabit connection.
Am I wrong anywhere here? And is there a maximum capacity HDD that is supported in my model MBP as a second hard drive?
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UPDATE: I purchased a dual hard drive adapter for swapping with the optical bay, and installed it in my machine with the original 500GB SATA III that Apple shipped with the laptop. So far everything is working just fine, and I have two working hard drives in the machine (triple booting Lion, Yosemite, and Windows 7 via Boot Camp). The second HDD is displaying 6.0 Gb/s link speed, with a 3.0 Gb/s negotiated link speed. I understand that even though the HDD is SATA III, it's dropping down to SATA II speed; but so far there aren't any problems with the disk running, and I can boot Lion from it without any issues. Will post any further developments.