For a little less than a year I have had a 2TB Crucial MX300 on my 5,1 running via PCIe Caldigit FASTA cards. In fact, I have 3 addition additional SSD drives on this system with two PCIe cards. The 2TB is the boot drive and had been running Sierra from the beginning. A couple of weeks, I started crashing at random intervals. Sometimes the screen would just go black, other times it was the spinning ball. I suspected a software issue. But there were no crash logs to follow up, so I tried various things. To eliminate hardware issues, I started running from my alternate install on WD 6TB black drive. No problem there apart from the very slow boot and overall performance. It took a day to keep that drive current, will have to stay on top of that backup. Then I started to see sporadic drive disappearance of the 2TB SSD, which did not cause a crash running from the other drive. That and the lack of cash logs finally triggered my grey cells. I checked the firmware. It was at M0CR031 instead of M0CR050. Ended up getting the updater on an ISO, then moved the SSD to the Intel controller, updated, and put everything back. No more issues. My hypotheses: a bug in the fw created long timeouts and unresponsive behavior during block reclamation that made the controller think, the drive was gone. I tend to be down to 15% free space on the system drive. But trim should handle this and never created an issue. 15% is > 300GB. In any case, outdated SSD firmware can cause problems, even if there was no major OS update! Crucial does not have frequent changes, so I do not tend to look.