Hi, I had the same thing happen. I've owned my second hand 5D Mark iii for several years. Love it, no problems, always worked great. Recently, I introduced a brand new Delkin Advantage SD UHS-I Memory Card. I have never used a card in the second slot before, but as a long time pro I really want the back up. Camera off when inserting, turned on, formatted SD card, CF card that had been in camera still there. I took about 15 shots, write speed seemed fine for my use. Cool. Set to go, right?
Later, I wanted to take a few shots. Thankfully it was not a job. Camera would not power on. Turned switch on and off, nothing, took main battery out, and back in, still nothing. A bit later I put a different battery in, it turned on. 🙂 Then, I turned it off, and on again and it would not power on. 😞
I googled, and found this post. I took out the battery, clock batter, removed the lens, and all cards. Let it sit for 30 minutes, replaced both batteries, it still would not power on. Being busy from just returning from out of town, I let it sit a few days. Before I called Canon, I thought I would put a battery in and check it. Yep, it came on! I have since powered it off, on, off, on, taking a few shots several different times. So far, it is still working. I am not totally trusting it though. Thankful for a backup camera. May need another one.
I am posting this because the previous posts really helped me. If I find out more defininte information, I will post it. 🙂
