I discovered this when removing OSD's TX pin - APM 2.5 fails to boot if OSD is disconnected or it's TX pin disconnected. (MinimOSD)
By fails to boot - i mean no servo movement in manual mode, nor any movement when switching to other modes.
if booted on USB, then connected to servo power, everything will work. (except OSD)
The OSD is connected using the small to dupont-5-pin connector. - and yes, I did try to remove the fifth pin (not sure why APM2.5 has it.)