I've been trying to find an answer to this problem, but I'm not good with electronics, and so I'm hoping there's someone out there who can provide a solution that I can understand.
So, a few weeks ago, I bought a pre-owned 2DS from Game Stop. It didn't come with an SD card, so I bought a 16GB San Disk micro SD card that came with the appropriate adapter. When I finished the settings, I chose not to connect it to the Internet yet. It works well for the few weeks I've had it. Yesterday (Mentioning this on the off chance it is relevant), I connected it to the internet. At 3:30ish today, it stops my game, and says something about write protection, and forces me to update and restart the 2DS. In response, I turn off the 2DS and take out the SD card and then check it. It was locked, so I unlocked it, figuring it was a simple mistake on my part. It still gave write protection complaints. I've fully updated the 2DS and even had the SD card formatted to make sure it was on FAT32, but nothing is working. Unfortunately, everything I have done was a blinded version of the solutions people have give to others who have experienced this and so I don't understand anything right now.
Thank you for your time.