There is a thread on the Behringer forum about this.
You can download: AOMEI partition standard edition 6.6
This tool allows you to format a 64 Gig card as FAT32 (and it still reads as 64 gig).
I did this 2 nights ago and it worked perfectly.
I did some sample recording to ensure the card worked properly.
I set my card to record 16 tracks and (in the x32 app) it shows I have 5 hours and 41 minutes of track time.
There are others stating on that other thread that they have done the same thing and have tracked over 4 hours successfully.
I just took my Lexar 256GB card from my 4K camcorder and formatted it last night FAT32.
X32 showed the correct hours (over 11h for 32ch, over 23h for 16ch).
I started a dummy recording (16ch) and am letting it run for close to 23h before I'll kill it.
It had already clocked over 6 hours this morning and time was still counting up/down for
time recording/time left on SD card.
It's a Lexar Class 10 rated up to 95MB/s read/write transfer speed.
I think Behringer will not officially support it due to SD card standards for manufacturers, but
if the user gets it to work, more power to them. They may support SDXC in future firmware
revisions, but I doubt it.
If it makes 23h+ recording, I'll post the results. Next gig, I'll slap it on record before we start and
not stop it to break it into sessions. That should give me over 6 hours of real time recording of
actual 16ch audio when I do that. I'll make sure there are no glitches in audio. Will probably
run it 32ch mode to stress it as much as possible even though we'll only have actual musical audio
on 16ch.