I attended a local class in lighting last year. Everything went well until towards the end when (using the same SD card) I also got an error message trying to shoot a few more at the photoshoot with softboxes, etc. I felt panic until I quickly turned off my camera; pulled out the SD card and saw that it was in "lock" mode. I don't remember if (but I must have at some point) I extracted the SD card and when I re-inserted it, it somehow pushed the small "knob" back down into "lock" mode. Just another possibility with SD cards.