Which SD card - the one in the drone or the one in the controller? Whichever one, I would try reformatting the card first and if that doesn't cure the problem, install/reinstall the latest firmware on whichever device is using that card.
Cards can go bad or weird. If formating does not solve the issue for good, replace the card. Cards are not expensive these days and not worth stressing about loosing important data on an unreliable card.