“…please make sure your SD Card isn’t full or write-protected.”
Well this is one weird problem I encountered after transferring my system data from the Nintendo 3DS to the Nintendo 3DS XL (after the built-in transfer process, I manually moved everything from the old card to the new card via PC). I only discovered it after I was curious as to why I’ve not received any StreetPasses and this message popped up when I entered the Mii Plaza.
I knew my card is not full, and I knew it’s not write-protected. And if you’re in the same boat then try the following simple fix that worked for me:
1. go into Mii Plaza
3. deactivate StreetPass and SpotPass
4. back out to the Mii Plaza’s main menu
5. then quit Mii Plaza completely (Home, X, A.)
6. start Mii Plaza again (no more error message)
7. reactivate StreetPass and SpotPass
Hope this helps and do let me know if this works for you.