Error 80 on 5DM2 !

My 5DMII is giving the "err 80" after the battery died in the middle of taking a photo. I could hear that the shutter did not fully cycle. I subsequently did a hard reset by taking out the main battery and replacing the date/time battery. The camera now works flawlessly when there's no memory card inside. The shutter cycles fully and everything works beautifully.

However the moment there is a CF card in the error 80 comes back. The CF card was empty and had been previously formatted in camera. I later tried reformatting the CF card on my mac as well to no avail.

Has anyone had this issue before? Would a diagnostic tell me what's happening? Any ideas are appreciated.

Have you tried a different cf card? Always format the card in camera only.

Yes I've tried with multiple CF cards

How long does your battery hold a charge? If it's not lasting long, I would replace it with a new battery, and see what happens. And I personally would not buy a 3rd party battery. I'd cough up the money for the Canon OEM battery. I've always had better performance with the OEM batteries, and in my case only had real problems with near dead Canon batteries or 3rd party batteries. Others may have had the opposite experience. I can only speak for myself.

I don't have any knowledge of diagnostic tools available to the general public. If this continued to happen after getting a new battery, I would send it to Canon service.