I was curious, so I went to the GoPro website. They say both micro SDHC and micro SDXC cards are supported. Amazingly, they suggest that when you see SD error on the camera, that you backup your video, then format the card, deleting all the video on it.
Did you insert the card in the camera when the camera was off? If not, try that.
https://gopro.com/support/articles/no-sd-or-sd-err-on-cameras-lcd
