Canon EOS-1D X Mark III Advanced User's Manual page 756

Conditions requiring card formatting
The card is new.
The card was formatted by a different camera or a computer.
The card is full of images or data.
A card-related error is displayed (
Low-level formatting
Perform low-level formatting if the card's writing or reading speed seems slow or if you
want to totally erase the data on the card.
Since low-level formatting will format all recordable sectors on the card, the formatting
will take longer than normal formatting.
Card file formats
CFexpress cards are exFAT formatted.
In movie recording, movies exceeding 4 GB are recorded as a single file (which results in a
movie file larger than 4 GB) instead splitting them into multiple files.
Caution
It may not be possible to use CFexpress cards formatted with this camera in other
cameras.
Formatting or erasing data on a card does not completely erase the data. Be aware
of this when selling or discarding the card. When disposing of cards, take steps to
protect personal information if necessary, as by physically destroying cards.
Note
The card capacity displayed on the card format screen may be smaller than the
capacity indicated on the card.
This device incorporates exFAT technology licensed from Microsoft.
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