These guys are fruitcakes. All you really want is to read your SD card, access your SD card again. I just went all over the NET and could not find a solution for this problem. Then, I just did this simple fix, and it worked for me on my Galaxy S4 Active: I inserted a new SD card, and it would not read it, mount it, etc. You know it. Then I read everywhere, that the standard Filesystem is NTFS, which did not work for me. I took another NTFS SD Card, and my G4 would not read it. So I copied the files off and reformatted the SD cards to (default) FAT32 (with the single file 4GB size restriction) So what? I'll just put several 4 gig files on the card, no problem.
After reformatting these SD Cards, the Galaxy4s would immediately read them, and gave me an option to reformat the 8Gig SD Card even, which I did. It came back with 7.4 Gigs, even more than the reformatting on the computer. This proves to me, that the Filesystem is FAT32 and not NTFS like some fruitcakes declared earlier. peace out.