Canon EOS-1D X Mark III Advanced User's Manual page 60

2.
Insert the card.
The card in the left card slot is [
With the card label facing you, insert the open side of the card (2) into
the card slot. Inserting cards the wrong way may damage the
camera.
The gray card-eject button pops out.
3.
Close the cover.
Press the cover until it snaps shut.
Card 1
Card 2
], and the card on the right is [
60
].