Unable to mount a 64Gb SD card on Qualcomm's SnapDragon 810 MDP smartphone
I was in contact with Intrinsyc Support, apparently "only FAT32 is supported for that size".

Now the problem was to format with FAT32, as Windows (arbitrarily) doesn’t support formatting FAT32 above 32Gb.
I tried this tool but it didn’t work: 
http://tokiwa.qee.jp/EN/Fat32Formatter/

What worked was:


Mount SD card on the phone
Settings -> Storage -> SD card -> Erase USB Storage


Apparently this doesn't just "erase", but also format the drive FAT32 (as I could verify afterward mounting it on Windows). The SD card is now mounted and running!