SD card unexpectedly removed
Sounds like an SD card problem to me. I've experienced similar problems a few times over the course of the years -- a card that has worked ok for some time has started to get "ejected" by itself. Changing the SD card has always solved the problem. .. Hmm. Except once but that phone turned out to be a very problematic case in a number of ways. 

So, try out changing the SD card, I would suggest.