Sometimes you may have issues with your SD card for your printer. The easiest way is to delete all the files on the card and test it with H2TestW. This is a Windows utility that is free that will test the entire space on the card for errors. This will tell you if the card is defective or not. This guide will show you how to use H2TestW. This page is for Windows users only. If you are not on windows, check out the F3 (linux) or F3X (mac) pages from 3ds.hacks (third-party site).
You will need to download the H2TestW file before proceeding with this guide. Download is at the bottom of the article.