SD cards have a tiny little tab on the left side edge (as seen from the front of the card). When this tab is in the LOCK position, a sensor switch in the camera tells the camera that the card should not be written to, i.e. the card is locked.
- Remove the SD card from the camera. Check the position of the locking tab and make sure it is not in the LOCK position. Reinsert the card, and try again.
- If you are positively sure that the locking tab is not in the LOCK position, and the camera continues to tell you the card is locked, the camera's sensor switch might be stuck.
- Sometimes, the locking tab pops out of its notch and goes missing. If you see the notch where the locking tab should be, but the locking tab itself is absent, you can tape over the notch with a tiny piece of thin adhesive tape. Be careful, if this is not done cleanly or the tape is too thick, the SD card might get stuck in the slot and you'll have difficulty getting it to eject.
- Inserting and releasing the card a few times, thereby jogging the sensor switch, may clear the problem.
- As a last resort, you can give the SD card slot a blast of compressed air from a can. Be careful because the "air" from the can might cool down the card slot to the point of causing condensation, so wait a while before turning the camera on again.
- If the problem refuses to clear, have the camera looked at by someone knowledgeable.