"If it isn't saving to the card when you put it in, either the card reader (in the camera) is dead, the card reader on your computer is dead, the memory card isn't formatted, or the memory card isn't compatible with your camera..."
Um, you left out some of the other causes... like bugs in the camera firmware that can be triggered by unhandled software errors... Someone once aimed the flash of their camera at my T3i while I was shooting a video about 12 feet away. It glitched my view screen during each flash. But what I didn't know is it put my camera in some kind of demo mode so that no image or video files were recorded until I removed the battery from the camera the next day. This weird mode was persistent across power-up/power-down cycles. It caused no noticeable changes in operation, just nothing was being saved to flash storage. After pulling the battery and then retesting the camera did it start working again. The camera could see the flash cards, display what was on them, and in all cases the cards were formatted by the camera AND were fully compatible... but once in this mode it wouldn't write files to anything, not even a newly formatted card. But it could read the cards while in this mode.
So add buggy camera software and modes to your list of reasons the camera will fail to record to flash.