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...."
I have this problem on my 3DS too, and i've tried other MicroSDs, cleaning the contacts (none are corrupted), and the oh so famous "reinsert." Yet my PC reads the SD card. So is it the SD or my 3DS? I really can't afford to get a new system.
- de Emily Grady
Same goes to me. When i reject the SD, my system read it, but when i try to connect Eshop, it say "cant access the SD"
- de Setiawan Ary