Canon’s EOS-1D X Digital SLR Camera benefits from a new firmware, version 1.2.1, which will add the disable option for Image size selection button. Also, this release will fix an issue when, under certain shooting conditions, Error 70 and Error 80 might occur.
This firmware version includes two files: one for Microsoft’s Windows OSes, and one for the MAC OS, each of the files including all the improvements from the previous versions.
Before you start, make sure that you use a fully charged battery or an AC Adapter so that the device won’t shut down during the update process, and opening the card slot cover while upgrading is highly not recommended.
So, in the downloadable file you will find an update file that must be copied in the root directory of a formatted CF card, and afterwards insert it into the camera. Select the “Firmware version” option from setup and press “Set” button.
After the firmware update screen appears, select “OK,” confirm selection, and, after the process is complete, turn off the camera and remove/reload the battery with at least 2 seconds gap, so that the process can take effect.
This said, follow the link below, and choose the proper link depending on your OS.
Canon EOS-1D X Digital SLR Camera Firmware 1.2.1