I've had a Nintendo 3DS for a while now (my old DS was on its last legs and I wanted to have a console to play FFIII on when I got there in my marathon) and one of the apps it came loaded with was a rudimentary music player. It comes with a 2G SD memory card so I was able to move my mp3s from my ancient mp3 player to the SD card through my laptop. I was then able to play the music fine in my 3DS, but every time I opened the music or camera app it would yell at me. The claim was that the SD card was set to write protected mode and therefore nothing could be saved to the card. I took it out and looked at it... Not write protected. I put it back in my laptop... Not write protected. I stopped caring at that point since it would play my music at least and went on with my life.
Nintendo released a patch on the 6th which added in a web browser and a game store. Great! Only problem, it wouldn't let me update with a write protected SD card. (Well, it didn't tell me as much, but I updated and it failed.) Mark had linked to an FAQ describing a bug in the update which broke the Ridge Racer game. (Want to keep playing it? Don't update your 3DS.) In the same sub-section was an SD card FAQ. It didn't describe my error at all, but it did advise that the SD card can only hold 100 music files per folder and 1000 folders total. That got me thinking... I have more than 100 mp3s, right? And I remember searching through trying to find a specific song I _knew_ I had on there but couldn't find it on the list. (Simple & Clean by Utada Hikaru) So I took my SD card out, split up the mp3s on my laptop, and tried it in the 3DS. Worked perfectly. Downloaded the update. Worked perfectly. Got my free copy of Excite Bike 3D. Woo!