Nikon D750 DSLR Camera (Body Only) - Manual (Page 285)

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In full-frame, zoom, thumbnail, and calendar playback, press the
L ( U ) button to protect the current picture from accidental
deletion. Protected files are marked with a P icon and can not
be deleted using the O ( Q ) button or the Delete option in the
playback menu. Note that protected images will be deleted
when the memory card is formatted ( 0 375). To remove
protection from a picture so that it can be deleted, display or
highlight it and press the
L ( U ) button.
Protecting Photographs from Deletion
A Removing Protection from All Images
To remove protection from all images in the folder or folders currently
selected in the
Playback folder menu, press the L ( U ) and O ( Q )
buttons together for about two seconds during playback.
L ( U ) button