Another pretty simple step: check that the card isn't locked. There should be a small switch at the side of the card which, shockingly, says "lock". Make sure the switch is the farthest away that it can be from that word, and you're good to go.

If the system tells you that the card is locked when it shouldn't be, use tape to hold down the lock switch securely.

While we're doing that, also look for any dirt or dust on the SD card or in the slot. If there is, try cleaning it with a soft cloth, or blowing it out with some compressed air.