Nikon D6 Reference Manual page 811

D
Folder and File Numbers
•
Once the folder number has reached 999, the camera will stop creating
new folders automatically and disable the shutter release if:
- the number of files in the folder reaches 5000 (or during movie
recording, if camera calculates that the addition of the number of files
needed to create a movie of maximum length would bring the total
over 5000), or
- the file number reaches 9999 (or during movie recording, when
camera calculates that the number of files needed to create a movie
of maximum length would result in a file numbered higher than
9999).
•
If there is space on the memory card, you will nevertheless be able to
continue shooting by:
- creating a folder with a number less than 999 and selecting it as the
storage folder, or
- changing the options selected for [Frame size/frame rate] and
[Movie quality] before recording movies.
D
Startup Time
Additional time may be required for camera startup if the memory card
contains a very large number of files or folders.
C The Photo Shooting Menu: Shooting Options 759