I am trying to figure out how to disable the internal flash yet trigger the external flash.
There is no need to do anything to disable the internal flash. In A,S,P, or M modes, the internal flash is already disabled if the flash door is shut. In A,S,P,M, it will not pop open and flash like it does in Auto mode (or does in the scene modes, which are also Auto). The internal flash is not "auto" unless in Auto mode.
In A,S,P,M modes, if you want to use the internal flash, then you just open its door, and then it will flash with any shutter button, every time. If not wanting to use it, then leave the flash door shut, and it will stay shut. The flash door is the On/Off switch that you seek.
This is from page 95 of the D5500 Reference manual page (much larger and more complete than "User manual", free at
Nikon | Download center | D5500 )
When you say "disable internal flash", I think you mean with the menu under the Flash Compensation button, where it says "Flash Off". That does not just mean the internal flash is off, it means
all flash is off, meaning no flash sync trigger at the hot shoe. The hot shoe radio trigger would not receive any trigger signal then either, which would explain a lot in your description. That Off menu will NOT be necessary in A,S,P,M modes if the internal flash door is simply shut, but Flash Off does mean Flash Off.
FWIW, it really needs to be M mode with the B800 manual flash, or with any external manual flash.
S or P modes can change the f/stop, which the manual flash cannot deal with the camera changing exposure settings.
No Auto ISO either, same reason.
Even A mode can change the shutter to be faster than maximum sync speed (extremely unlikely indoors though).
The camera normally knows about any hot shoe flash it communicates with, and will not mess it up as much if flash is M mode, but camera does not know about presence of a manual external flash.
Manual flash is simply done in camera M mode.
This lets you set aperture to match the flash power level, for your exposure.
M mode also lets you set shutter speed. Flash does not care what shutter speed is (so long as it does not exceed maximum flash sync at 1/200 second). Because flash is much shorter than the shutter speed, the shutter just has to be open to pass it, but shutter speed does not affect the flash exposure.
But flash pictures can be two exposures, from ambient, and from flash. Shutter speed definitely affects ambient, but not flash (which is another control available to you). Ambient is normally insignificant level indoors, which is one reason why flash is used. But you can choose to set shutter speed slow, like 1/60 second, or even slower, to let in more ambient light. Or you can set shutter fast, like 1/200 second, to keep out the ambient light. Lighting ratio so to speak. Photographers choice, which becomes important when understood.
And worse, incandescent ambient is often orange, one good reason to keep it out. A slight bit of warming might be good sometimes, but orange is orange. This is why low ISO is most desirable for flash... high ISO just lets in very much ambient.