Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 382)

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You can copy the images recorded on one card (save duplicates) to the
other card.
1
Select [Image copy].
Under the [ 3 1 ] tab, select [ Image
copy ], then press < 0 >.
2
Select [Sel.Image].
Check the copy source card’s number
and the target card’s number and its
remaining capacity.
Select [ Sel.Image ], then press < 0 >.
a
Copying Images
3 Copying a Single Image
Movie Files Exceeding 4 GB
If you are copying images from a CFast card ( g ) to a 128 GB or smaller
CF card ( f ) formatted in FAT32, movie files exceeding 4 GB cannot be
copied.
Movie files exceeding 4 GB can be copied between a CFast card ( g )
and CF card ( f ) as long as the latter has a total capacity of 128 GB or
greater and is formatted in exFAT.
The copy source is the card selected for [ Record/play ] or [ Playback ] under
[ 5 1: Record func+card/folder sel. ].