I've got a Sony "64"GB UHS-I uSD current in my n3dsxl and it works fine stock. [EDIT] FAT32/32k clusters[/EDIT]
I'm waiting for a couple additional "64"GB uSDs(SONY again since good price/perf, used the ealier extras on a different project so i didn't have any spares for when gw delivered) to show up(probably saturday) as I'm going to archive the current "64"GB for the time being and run the gateway w/a fresh blank one.
Anyone else find the WiFi system management thing flakey/useless? (well I know already that it's INCREDIBLY sssslllllooooowwwww) I just ask as I attempted to backup the original (cheap --- Nintendo 4(8?) GB uSD) via that before cracking it open to swap in the 64GB but it always ended up crapping out and given how damned sssssllllllloooooowwwwww it was I ended up just cracking it open(major PITA although I understand it's easier after violating it the first time

(small joke)) and backing it up via a USB stick.
All I can say is than CN is the best $10 that I've spent on the n3ds so far, especially that I really decided not to buy one until release day(and bestbuy was useful for once a 9.0 black...) and just bought CN for ninjhax and it looked like I might actually play it a little... guess the time was post-ninjhax scalping and pre-gw hypetrain exploit.
WiiU: ...and so the wiiu exploit people, only boarded that train last month as well(originally looking for wiis but new ones were of questionable origin/region and older ones insanely overpriced(not pre-hacked even, even though that's trivial)) and it shipped with 5.3.1 which I just let go ahead and update to 5.3.2 IIRC. (Might've been a slightly older fw but I'm certain that it was 5.x.x and general consensus was vwii hacks still worked, no certain wiiu hack so go ahead and update, so I did... which reminds me that I need to do some usb loader gx testing some time...)