This is also happening to friend of mine with 4.3 stock deodexed rom.
The card is fat32.
When i meet him again i ll switch my card with his to check, as my exfat card doesnt have any issue.
Interesting
A dev sent me a few time ago an issue about some phones messing up with external storage
Basically, it gets mapped to /dev/block/mmcblk1 and not mmcblk1p1
Now, if you can try his card on your phone, it will be of great help
That way, if his card can be mounted in your phone under recovery, than, definitely, something related to bootloader/device or some partitioning ****ed up by crappy roms
Let me know if you can try it