1. All other cards work fine, so it should not be the camera.
2. Same as #1.
3. Same as #1.
A few suggestions:
1. If you have another DSLR, perhaps you can see if that can read the card in question.
2. Do a hard reset of your 7D by removing battery and backup battery. Leave these out and let the camera sit for 30 min. Then reinsert the batteries and power up the camera.
3. Check firmware and upgrade to latest version, if you haven't already.
Hope it helps.