Just to be sure (perhaps this is obvious... but just in case it's not), you should format the card (even though they may claim to come pre-formatted) after inserting a new card into your camera using the camera menu. This is true of any "new" card and also true of any card that was last used in a different camera.
Camera's lay down a menu structure that they need to have. So formatting isn't just doing a low-level format (like a disk), it's also establishing the directory structure.
What brand and type are your cards and are these same cards confirmed as being working cards in other cameras?
I'm thinking either the card slot works or it doesn't. So if specific cards are giving errors... I'm leaning slightly in the direction that this is probably an issue with the card rather than the camera.
I typically use SanDisk brand Extreme Pro cards - 16 GB UDMA 6 cards -- which have had a perfect performance record for me so far. (note: I don't have a 7D, but I know the 7D can handle substantially larger cards than 8GB.)
