Problem with external SD cards unmounting - Possible Solution ... I've had my Maxx 2 for a year. For the longest time the external sd memory card, a 128 gig Sandisk, worked flawlessly. As I added more files it suddenly became unstable and would dismount the external card on it's own. To the common user you would realize music and music lists no longer showing up, or photos no longer appearing, and other data stored there seemed lost. I was going to do an advance reformat of the card in the phone to insure a possible flawless "connection". I also read about making the external SD card part of the main memory storage, like an extended internal memory card. One other repair suggestion was to move large apps back to the main storage.
The first thing I did was to copy all the content to my main computer off the 128g sd card by installing it directly in a matching port on my main computer system. As I was duplicating the card, or backing it up, I discovered damaged files that would not transfer. 3 to be exact. One was even a video the phone took, and the other 2 were damaged photo files. During the back up a warning appeared questioning the validity of those files. I removed them off the external card and installed it back in my phone and I have NOT had any issues what so ever with the card unmounting. It also sped up the apps that had their data stored on the external sd card. Suddenly my complete photo library appeared, along with my playlists and music. I've done a lot of reading and no where did anyone resolve the problem with a simple check of all the files being properly operational. So clean out any files that may be damaged and see if it cures your issue.