Maybe this can help:
"For anyone that is still having this problem (rip the video poster)
this is what worked for me, my 3ds wouldn't detect my micro sd:
"Now that I got some help on this, I need to pay my dues. Formatting properly and ensuring it was primary and not logical was not enough for me. I needed to do one more step to get it to work; using MiniTool Partion Wizard (downloading this software was sketchy for me, it forced a download of chromium, (not google chrome?) and I believe is installed a virus, ty windows defender) I changed the Partion Type ID to Fat32 to from what it was, Ex-something, but I'm sure it will work for any Partion Type ID your card comes with.
I'm using a MicroSDXC U3 Samsung EVO Select 64 GB with a Samsung SD Adapter "Samsung 64GB 100MB/s (U3) MicroSDXC EVO Select Memory Card with Adapter (MB-ME64GA/AM)"
in my Super Smash Bros. Edition Nintendo 3DS XL. I'm so glad that I found a fix, I would've been so !#^&@@ had I bought a 64 gb micro sd that didn't even work for my 3DS...."