I had S4 myself and that phone had a problem with exfat. It format drives with corrupted exfat, which was not compatible with any other device. But that happened on stock. Check following steps:|
1) check if folder /system/su.d/ exist and if there is 00mountsd inside
2) check log file /data/local/tmp/exfat_mount.log
3) try to mount with modded version of Paragon exFAT, NTFS & HFS+ (4pda) -> modded version supports exfat!
4) if you formatted your sd card with Samsung phone, try to format on pc and try again
If those steps don't work, can't help you either.