I have a Samsung Galaxy S5 SM-G900W8 running Android Lollipop 5.1.1. It has a Kingston 64 gig MicroSD XC 10/64GB card in its card slot formatted as exFAT.
Unfortunately, I can't copy to the SD card via USB. The phone mounts as a drive letter fine...
Here's what I did to solve my problem -
1) Took the card out of my phone and put it in my PC
2) Backed up the card
3) Downloaded Rufus
4) Opened RUFUS and reformatted the card with a 'quick format.' Selected large FAT32, ...