Nikon D6 Reference Manual page 1072

Monitor
This file cannot
be saved to the
destination
memory card.
See the
camera's user's
manual for
details.
1020 Alerts and Error Messages
Alert
Top
control
panel
—
Problem/solution
Files 4 GB or larger can only be saved to
memory cards formatted for exFAT. They
cannot be saved to cards formatted for
other file systems, such as FAT32.
Use a memory card with a capacity over
32 GB formatted in the camera or keep file
size to under 4 GB.