5D mark I
So recently I've been getting some memory problems on my 5D. It started with a fairly new Sandisk Extreme IV 4gb card. I was using it fine in my old Sony DSLR but after using it on the 5D, after about 3-4 formats I started getting "No Image" messages on the screen. I would format the card, then take a few shots then during playback the "No Image" message would appear again and then the red light on the back of the camera would stay on and not go off without turning the camera off. So I went and bought a new 4GB Sandisk card today, same one, put it in the camera and I select Format from the menu and it says "Cannot Format" then screen goes black and eventually the message "CF card not formatted". Don't know if it's the camera or not (it's a refurb I just got a month ago) but the other cards I have work fine.
Edit: card reads and writes fine when mounted on my mac.