EOS 5D Mark IV No card in camera / Card cannot be accessed

[josephineadair]

I am a wedding photographer. I have two 5D Mark iv’s. Both of them experience the same issues. The first issue being that they will say “No card in camera” when there is a card inserted. The second issue is they will say “Card cannot be accessed, reinsert or format.” Does this sound like bad cards, or camera problems? My cards are not old at all, and it’s very stressful. They will say these messages after the cards are already formatted to the camera so I don’t think that’s the issue. Oftentimes reinserting does work, but sometimes either one of these error messages will come up again after having reinserted. It’s very stressful for photographing weddings. I really need help here. All of my colleagues have the same camera and use the same cards, and have never had these issues. I wish I could just chuck all my cards and buy all new ones, but it just keeps happening. Please help. 

[rs-eos]

How old are the cards? Are they formatted before each event?

Also, what specific brand and size (e.g. full sized vs. micro and adapter)?