Throwing this one out there hoping the google juice will help people with the same problem.
I’ve tried installing an SSD into the optibay my 2011-era Macbook Pro with no success: the SSD stalls on writes rendering the whole thing useless. Recently I went through the Crucial selection process, ordered one that was supposedly suitable for my mac. Still the same problem. Musch googling, finding much chatter about the SATA in the optibay not being up to snuff and to use the main drive bay instead. Did that. It worked!
So the solution to a halting SSD in a 2011-era Macbook is to use the main drive bay.
I did and it screams. Instant everything. w00t!