When you are using an external flash and do not want the internal one to fire, you can turn the latter off. Note that turning off the internal flash and folding it down are two different things. Turning off the internal flash means that the internal flash will not fire even though it is raised until it is turned on again. On the other hand, folding down the internal flash only means that we do not want to use it. As long as it is raised, the internal flash is ready to use.
To turn the internal flash on or off, follow the procedure below:
Then, press the right arrow of the multi-selector to select it.
For some unknown reason, Nikon decided to do the following strange and very confusing thing:
The selection is exactly the opposite of what you want: select Off if you want the internal flash to fire, and select On if you do not want the internal flash to fire!
Make your decision, use the up or down arrow of the multi-selector to highlight the option, and use the right arrow to make a selection.
Note that once this option is chosen, it will stay even after the camera is turned off. Therefore, you need to go through the above procedure again to reverse the setting..
An Important Note
After the internal flash is turned off, it will not fire even though it is raised and the scene is not illuminated sufficiently. In fact, once the internal flash is turned off, you are supposed to use an external flash. Check the external flash pages for the details.