The only way an R4 can stop reading SD cards is if you had an early model, which has a spring inside for ejecting the SD card, and the spring happens to get stuck or something. Though mine has a spring in it because I bought it over a year ago, I always pulled out the SD card instead of ejecting it because I never knew you could just push it in to eject it until a few weeks after I got the thing. I'm still pulling it out like what people have to do with the newer models just to be safe.