[BrianG]

Disabling the BEC via removing the red wire doesn't really disable it, it just keeps it from powering external devices like servos, fans, rx, etc. Even if you do remove the red wire, the internal fan still runs off the internal BEC. The only way to put the fan on an external BEC would be to unplug it from the PCB and route the wires outside the case to your external BEC. The internal BEC would just be powering the brains PCB at that point.