Nikon D750 DSLR Camera (Body Only) - Manual (Page 147)

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When two memory cards are inserted in the camera, you can use
the
Role played by card in Slot 2 item in the photo shooting
menu to choose the role played by the card in Slot 2. Choose
from
Overflow (the card in Slot 2 is used only when the card in
Slot 1 is full), Backup (each picture is recorded twice, once to the
card in Slot 1 and again to the card in Slot 2), and RAW Slot 1—
JPEG Slot 2 (as for Backup , except that the NEF/RAW copies of
photos recorded at settings of NEF/RAW + JPEG are recorded
only to the card in Slot 1 and the JPEG copies only to the card in
Slot 2).
Using Two Memory Cards
A
“Backup” and “RAW Slot 1—JPEG Slot 2”
The camera shows the number of exposures remaining on the card
with the least amount of memory. Shutter release will be disabled
when either card is full.
A
Recording Movies
When two memory cards are inserted in the camera, the slot used to
record movies can be selected using the
Destination option in the
movie shooting menu ( 0 319).