Canon EOS-1D X Instruction Manual page 241

An increase of the camera's internal temperature may cause movie
shooting to stop before the maximum recording time shown on the
preceding page (p.249).
Even if [Record func.] is set to [Auto switch card], the card cannot be
switched automatically during movie shooting.
About Full HD 1080
Full HD 1080 indicates compatibility with High-Definition
featuring 1080 vertical pixels (scanning lines).
3 Setting the Movie-Recording Size
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