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Make sure the cards are not too large for the camera's memory card slot to write to. Coming back with nothing on the card sounds suspiciously like that, if not exactly definitely that sort of problem.
Then, consider whether the cards were Write-LOCKed. Check the LOCK switch on them, in short. To be fully usable, the card should have the LOCK switch in the direction of the base of the arrow next to the switch, not in the direction of its point (which is the LOCKed position.)
After that... your camera might well have a defective memory card slot. In which case, considering it is about 6 months old (less than a year old) you should at that point call
and put in a failed-within-warranty claim on the camera, for free-of-charge repairs to it under Canon's 1-year basic warranty provisions. (Note: it's good to have proof of the purchase date handy when you make that call. Whether a sales slip, or credit-card records, proof of purchase date may be necessary to present.)
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Well. What I meant by 'too large' is, that in some Canon cameras (and other devices) the SD card slot can't handle the cards with total capacities above 4 GB... the SDXC and SDHC cards, that is.
If one card came out of another 7D camera and worked there, that proves the case: the memory card slot in your problem camera is defective somehow.
And, my advising you to send it to Canon for warranty repair, free-of-charge, stands all the more, so.