Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 60)

2.
Insert the card.
Card 1
Card 2
The card in the left card slot is [ ], and the card on the right 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.
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