Have a 32gb card put it in my s4 and it won't mount. Tried to reformat it upon inserting it and it just keeps saying it is blank or has unsupported files...
Is there important data on the SD? If not, try formatting it on your computer (using FAT32). You might want to do the full format, rather than the quick format, in case there are any bad sectors.
You can buy a USB adapter key that will plug into your USB port on your computer... it will show up as a new drive... then just format the card as stated above using FAT32 format type.