Preamble (Skip if looking for solution):
I’ve been using a Motorola Moto G 3 (2015) for a while now, for a few, ultimately unsubstantial reasons. First, there is some pleasure in using a well-built device made by a market underdog. (I’ve used phones from HTC, LG and Kyocera in the past) And second, I had (erroneously) thought the price/performance increase of mobile phones have pretty much plateaued in recent years. My experience with low-cost underdog-made handsets have been pretty good, latest of which was the HTC 816 which I have very happily enjoyed its ginormous screen until I lost it while running a race. The next phone I chose was the Moto G 3, which came with good reviews. The reviews are mostly accurate: It has decent performance for its price, screen size and batteries are good, and the 8GB internal flash is lacking, but not unbearable either. However, a big problem with this phone (and other low-end Motorola phones as well, apparently) is that, after some months of usage, it displays an error message saying the SD card was removed unexpectedly. After which the SD card will no longer reliably function, regardless of frustrated reboots.
So, solution:
Use a class 4 8GB SD card.
Class 10 cards are easy to find, and capacities of 16GB and up are very cheap now. However, many low-end Motorola devices can’t seem to handle class 10 cards reliably. (I’ve tried 64GB and 32GB cards, both of which caused the same problem after a bit)
I’m not sure if a class 4 card with larger capacity will work, or if class 6 will work. Some searching online seem to suggest use of class 4 though. I am currently using a class 4 8GB card, and it has not caused me problems so far.
YMMV, so please let me know if you discover anything new 
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