SDCard very hard to read
How to fix the Nintendo 3ds SDCard very hard to read . I put in my sdcard into slot but 3ds display "could not detect the sd card"? Is the sdcard problem or the sd card slot problem
Nintendo handheld device released February 2011, identified by model number CTR-001.
I would try using the SD card in other devices if possible to see if they read it. If they do, then it is likely not a card issue and instead is and issue with your 3DS.
First, you should take a flashlight or bring the device under some bright light and inspect the cartridge slot and all the contact points to see if there are any issues (bent contacts, broken contacts, anything that looks out of place).
If everything seems to look okay you can try cleaning the slots gently with a little bit of rubbing alcohol. Usually it's best to use a small q-tip but if the slot is really skinny you can always try a bit of folded up paper towel, etc.