If the SD card is unable to be read, first check if the card is sliding smoothly into the card reader. There may be debris or other small particles blocking the card. If there is, remove the debris and replace the SD card. Make sure the spring lock is properly triggered. If the SD card is still nonfunctional, the SD card itself may be broken/defective, and may need to be replaced. Also, refer to the troubleshooting guide for more details on any other SD card issues here.