Ok...I think the issue is the Bootflag thing. My SD card is bootable, so I think it is doing exactly what the USERGUIDE explains will happen.
Formatting the card on the computer is fine, but putting it in the camera will not allow camera to come on.
I thought maybe I might be able to switch off the bootflag in order to use the card. Specifically, it is an Eye-Fi card. If there is an easy way to switch back and forth, I am willing to try it. But we do have a need to use that card quite frequently. I found a few discussions about a way to toggle it, but they didn't work for the 5D Mark III.
Formatting the card on the computer is fine, but putting it in the camera will not allow camera to come on.
I thought maybe I might be able to switch off the bootflag in order to use the card. Specifically, it is an Eye-Fi card. If there is an easy way to switch back and forth, I am willing to try it. But we do have a need to use that card quite frequently. I found a few discussions about a way to toggle it, but they didn't work for the 5D Mark III.