BE AWARE !!!!!
If you have a 1dMKIV and you attempt to update your firmware with an EYE-FI card in the 2nd slot your camera will go dead as soon as you start the firmware update, even though you have the new firmware code on the CF card in the primary slot. This happens whether you do the firmware update by placing the firmware in the root of the CF card or if you use the EOS Utility to do the update.
Canon CPS was not aware of this issue and I replicated it in 2 separate bodies, so much so that they sent me shipping labels to have my bodies checked.
To get your body to turn back on, remove the battery, remove all cards, put the battery into the body, put ONLY the CF card in the body, power it on and your body should now complete the firmware update. If not simply power off and restart the process
