Background: I have a MacBook Pro (late 2011) with a SATA III 6Gb SSD installed in place of the optical drive. Sources say this configuration should work, but I've been having problems (read/write failures, spinning beachball, etc) of exactly the sort you would expect if a SATA II bus was trying to operate at SATA III speeds. I'm beginning to suspect that my laptop may have the issue reported for early 2011 MacBooks that made them unstable with 6Gb SATA III. The best solution would be a SATA 2.0 drive, but the drive I have is already in hand.
Is there any way (preferably through software) to set the SATA Negotiated Link Speed to 3Gb instead of 6Gb?
Edit: The SSD in question is a Seagate/Samsung 850 EVO (MZ-75E250B/AM), if the solution is drive-specific.
Best Answer
So, having verified the drive is, in fact running at SATA-III speeds, it looks like this post describes doing what you need. Be aware it requires a DOS environment to run, so you'll likely want FreeDOS with the .iso loaded already.
I haven't used this, so I am unable to verify results. Attempt at your own risk.