Canon MARK IV User Manual (en)

Selecting the Card to Record or Playback
When [Auto switch media] is set, the card for recording will switch from 
f to g to u.
When [Rec. separately] or [Rec. to multiple] is set, the image will be 
recorded under the same file number to both the CF and SD cards. Also, 
the top LCD panel and viewfinder will display the number of possible 
shots of the card having the lower number. If one of the cards becomes 
full, [Card* full] will be displayed and shooting will be disabled. If this 
happens, either replace the card or set the recording method to 
[Standard] or [Auto switch media] and select the card with remaining 
space to continue shooting.