Here's how I formatted the card: I made a backup of both internal and external memory, shut down phone, pulled the battery and memory card. I then installed the 64GB card and restarted. This is where everyone gets the error message, including me. It says something like "unable to mount card" or something. So for me, all it took was another restart (with a battery pull?) and upon booting up, the phone had reformatted the card. That was it - works great.
Now if this doesn't work for you, you can get your pc to do the reformat. The problem here is that Windows no longer supports reformatting FAT32 for volumes larger than 4GB ( I read that somewhere). The workaround is a freeware program that WILL reformat 64GB. Do a search(FAT32 reformat). I found many such programs. I hope this is helpful.
I just wanted to echo the above, this worked for me as well for a 64GB card.
I backed up everything, internal and external, "just in case".
I then formatted the SD card to FAT on the Mac, (since my Mac has an SD Slot handy here at work).
I then brought the phone up, and checked in Settings -> Storage -> SD card -> Total Space.
Indeed, it shows Total Space of 59.45GB, as you would expect.
I then connected the phone to my Windows 7 box, and let the drive mount as usual.
It was properly seen under Windows, and sees all the space we would expect, as available.
I then copied back all my data files from the back up of the external flash, to the new flash, and rebooted.
Everything is seen, I can browse/play with all files on the flash.
So, long story short, the whole key here, is to format your SDXC as FAT32, either by the Mac, or by a util under Windows, and you should be set to go.
