Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 52)

Installing and Removing the Card
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When the access lamp is lit or blinking, it indicates that images are
being written to, read from, or erased from the card, or data is being
transferred. Do not open the card slot cover during this time.
Also, never do any of the following while the access lamp is lit or
blinking. Otherwise, it can damage the image data, card, or camera.
• Removing the card.
• Removing the battery.
• Shaking or banging the camera around.
• Unplugging and connecting a power cord
(when household power
outlet accessories (sold separately, p.485) are used) .
If the card already contains recorded images, the image number may not
start from 0001 (p.206).
If a card-related error message is displayed on the LCD monitor, remove
and reinsert the card. If the error persists, use a different card.
If you can transfer images on the card to a computer, transfer all the
images and then format the card with the camera (p.74). The card may
then return to normal.