Super weird issue going on. Upgraded my 13" 2011 MBP with a Samsung 840 Pro SSD to High Sierra. (Automatically upgraded to APFS with TRIM enabled) Last night it wouldn't boot showing only a flashing folder/question mark. Did all of the usual stuff PRAM, Recovery Mode, repaired permissions, etc. Nothing worked; it just won't boot. Took the drive out of the machine and put it into a USB enclosure to see if it would boot externally and it does! I have verified that there is nothing wrong with the drive. I can boot it when it is attached to the machine externally and use it as normal. I can even make it the startup disk when external and it works flawlessly. As SOON as I put it back into the machine, the computer says it can't find the startup disk. Has anyone ever come across this before or seen similar behavior with 3rd party SSD's??