Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 388)

L Erasing Images
388
You can erase all the images in a folder or on a card at once.
When [ 3 1: Erase images ] is set to [ All
images in folder ] or [ All images on
card ], all the images in the folder or on
the card will be erased.
3 Erasing All Images in a Folder or on a Card
To erase all images including protected images, format the card (p.74).
When [ All images on card ] is selected, the images in the card selected
under [ 5 1: Record func+card/folder sel. ] with [ Record/play ] or
[ Playback ] will be erased.