Since the Marshmallow update, it constantly mounts/unmounts or wants to format the SD card. I know it can still read it as it sometimes displays the contents, but usually not long enough for me to attach a file from it. I've taken the card out and rebooted..reinserted the card and rebooted a few times but it still does this. Basically using a SD card is no longer an option
To the other comment I'd definetely try another card. You'd be surprised that the card could actually be bad vs. the contacts on the device. It is worth a shot. Amazon seems to have the best prices on memory cards.
I am curious about this as well. I have a Galaxy S5. When the Lollipop upgrade (a disaster) was released, suddenly my 32 gig SD card went "bad". Replaced it. Marshmallow comes along. Again, my 32 gig card suddenly goes "bad". What is happening with the upgrades that kills the SD cards?