I spent hours looking for a solution to my issues with my 64gb Micro SDXC card that my girlfriend purchased me for my birthday.
From what I understand only capacities up to 32gb are officially supported. But thread after thread informed me that users of my phone had successfully used one. When I first plugged in my SD my phone said "Preparing SD card" and then, after a brief pause, "SD Card safe to remove". The card wasn't mounting.
I tried inserting it into my SD adapter and formatting in NTFS on my laptop. Side note - whoever designed SD adapters and ports can go straight to hell. I was terrified of damaging my expensive new card since it got stuck in the adapter, and the adapter left 1mm of space outside my laptop. Holy crap.
After formatting in NTFS the phone informed me it'd have to reformat the SD card. Progress, I thought, until after the formatting message I had the same issue.
I thought maybe my firmware was the issue and spent a couple hours trying to figure out how to jailbreak. Long story short I gave up.