Nikon D300 - Manual (Page 335)

309
U
f9: No Memory Card?
At the default setting of [Enable release], the shutter can be
released when no memory card is inserted, although no pictures
will be recorded (they will however be displayed in the monitor in
demo mode). If [Release locked] is selected, the shutter-release
button is only enabled when a memory card is inserted in the
camera. Note that when photographs are being captured to a
computer using Camera Control Pro 2 (available separately),
photographs are not recorded to the camera memory card and the
shutter will be enabled regardless of the setting chosen for this
option.