Hi guys! Thought I'd post this here since there's little information about this exact procedure. Today I proceeded to swap the original Apple TS256C 256GBSSD with a Samsung EVO 850 250GB in my iMac 27" mid-2011 12,2. It's a pretty straight forward procedure and you can follow the instructions up to 4:10 in this video. The factory installed SSD lies behind the DVD drive. Once you get there, you simply remove the 3 screws holding the SSD brackets in place. Simply attach those brackets to your new SSD, screw them in place and reassemble the iMac. Before doing it, I wasn't sure if I would have to take out the logic board but there's no need to. Next time you reboot, enable TRIM support by going in the terminal and typing: sudo trimforce enable and follow the instructions until the computer reboots (it takes a little while). The performance difference is pretty noticeable on boot and application launch times, as well as when user switching. Xbench showed 100% better score than stock SSD and Black Magic reports around 495 MB/s write and 505 MB/s read, up from 120 MB/s write and 200 MB/s read. Very glad with the upgrade and hope this can help someone. Attached is a picture of the installed SSD before reassembly.