We had this happen to our 6D and it was the circuit board. Sent to Canon for repair and $400 - $600 later it came back working. I believe on this occasion they should have ate the cost, but it wasn't my camera and I didn't handle the return.

Here are some things we checked before sending in for repair:

Check that the card is formatted properly (WARNING!)

• Check that the card is formatted properly for your camera. Some cards come formatted differently (FAT32, exFAT, etc). Generally I would use a computer to check the card for anything i needed first then format in the camera using the low level format option. Don't format your stuff dude, that would suck.

Check that the card isn't damaged or in locked mode

• I understand locked mode shouldn't do anything, but why not check. Also, if you wrote on the card with a pen, or it is damaged then it might be the card.

Check another card

• Plop in another card that is compatible with your camera.

Make sure the card is compatible with your camera

• Cameras and cards are not all created equal. Some cameras can not use cards if they are 'too good' for your camera, and some cameras can not read or see cards that are 'too basic' for your camera bus.

  • Here are some basic guidelines for choosing a card that matches your camera:

  • Sandisk SD Card Recommendations

  • SD and Compact Flash Card Numbers Explained

  • (I will convert this to static content soon and set a reference, can't right now)

Check that you are using a Canon battery

• I know this sounds strange, but I recall we had problems with non-Canon batteries (el cheapo) and card read errors. Not sure why.

Power down the camera, pull the battery

• Power down the camera, pull the battery and wait a minute then reinsert

Check the card contacts and clean them

• The contacts can have bad contact with the camera contacts for a number of reasons. Clean the contacts properly and try again. Also, check the camera contacts in the slot using a flashlight to ensure there isn't anything like fuzz stuck in the port or damaged pins.

Call Canon Tech Support, Send for Repair

• Although there might be more things to check, at this point we thought it was time to get the manufacturer involved. So we called, then sent it in. Now it works.

Hope you get good luck and it is simple. Last note. Check the video record after you think it is fixed. Even after we got ours working temporarily we noticed it would have a write error in a short amount of time while recording video.