I had also picked up SanDisk Ultra, 64gb microSDXC for my S4. I had a 32gb sandisk ultra in there prior to the 64. The 32 worked fine, no issues whatsoever.
When I put the 64gb, occasional unmounting,etc. and also lost some pics and vids of family.
Pretty irritated, but after trying several different things it worked alright for a day or more, then I'd get an unmount...sometimes rebooting fixed it, sometimes it fixed itself. Either way, had no issues with the 32 of the same brand/type, only with the 64gb and that was unacceptable to me...shouldn't be that much work for a card to be so finicky in the phone.
I was able to exchange it out for a Samsung 64gb pro for about $15 more. No further issues.
At any rate, a lot of folks want to try different "fixes," like unmounting, mounting, restarting, formatting here, formatting there, inserting, uninserting back into the microSD slot...but the truth of the matter is- if the fix was that simple or universal, there would be one solution for all.
Bottom line is, if you're having issues or have had an issue, I would highly recommend first backing up any data (and on a regular basis that you would like to keep, in the event that the card decides to take a crap) as well as contacting Sandisk to see if they will offer a replacement. These items aren't designed to be "fussy" and "hit-or-miss" - obviously something just isn't right.
I suspect there will be further issue to address when Samsung updates the S4 to allow for apps to the SD card and more folks are out there trying to buy larger cards (ie 64gb from Sandisk)....
All the other Sandisk products I have own have never, ever given me issue...just that doggone 64gb.
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