Sorry in advance for the long description, this has been a real mystery to me (and Apple) and I'm hoping you guys can help. In summary, I did the optical bay HD conversion on my MBP. The hard drive in my optical bay (1TB Samsung 5400 rpm drive) keeps disappearing from my desktop (and from my computer entirely), which leaves my computer unusable (because my home folder is stored on that drive). I got some sort of S.M.A.R.T. error just before the hard drive disappeared. In detail, here is what has happened: I bought one of the new (still current) 2012 MBP's (2.3 Ghz quad core i7, 16GB Ram) and did the OWC optical drive conversion. I have a 250GB Crucial M4 SSD in my normal HD spot, and took the 1TB 5400 rpm drive out of my old computer (that was working fine for the past year) and put that in the optical bay spot. I have the newest OS (10.7.4) and applications installed on the SSD, and my home folder (containing my music, desktop, photos, etc) installed on the 1TB drive that is in my optical bay. Everything worked great for about a week, then suddenly the 1TB drive disappeared from my desktop (and my computer entirely) and the computer forced me to do a restart. The computer was not hot, being moved, dropped, nothing, at the time of this happening. When I tried to restart the computer, I only got a black screen. I tried everything including resetting PRAM, different RAM, putting in a new OWC kit, etc. Nothing worked so I took out the OWC kit, got the computer back to factory condition and took it to the Apple store. They said the logic board/ graphics card was bad so I exchanged it for another one. In the meantime while the new one was being shipped to me, I put the same OWC conversion (250GB Crucial M4 in the normal HD spot, 1TB 5400 drive in the optical bay spot) into my old 2011 quad core MBP. Everything worked perfectly! So I knew it wasn't the hard drive, etc. Then, after about a week (similar time period as the first time), the same thing happened. The HD in the optical bay spot disappeared and the computer was useless. But this time, the screen still worked (it wasn't just black). My new 2012 MBP replacement came the same day it failed so I took both drives out of that 2011 MBP and put them into my new replacement 2012 MBP. Everything worked perfectly again! I then put my old hard drive and optical drive back into my 2011 MBP (restoring it to factory condition) and IT TOO worked perfectly again. So this time, it must not have fried the logic board/ graphics card. Now, it happened today a 3rd time on my new, replacement 2012 MBP. Each time it happens, it happens about 1-2 weeks after I set everything up. I have a Temperature Monitor installed and this time I got a S.M.A.R.T. error notification (the notification may have been from the temp monitor, I'm not sure) saying something like one of the SMART sensors went offline or something, then a few seconds later the 1TB drive (that's in my optical bay) disappeared and my computer forced me to shut the computer down. This time, unlike the previous times, when I restarted my computer- both drives were there again, working fine, and everything seemed totally normal. I am worried this will keep happening and only get worse. From what I have gathered- I have ruled out RAM, the OWC optical bay conversion kit, and the computer being at fault. Maybe the 1TB SAMSUNG HN-M101MBB drive I have is starting to go bad? Or maybe it's not compatible in some way? Or maybe I have installed something improperly? I am hoping you guys can help me (and anyone else that will have this problem in the future). Thanks in advance!