Actually, I don't think it would hurt a thing. OP would connect a standard 1394 external port block to a serial port header with the same physical format. That would send the serial port to the external FW port. What would be connected to that? Therin lies the problem. Unless some external device is connected to that port, nothing will happen. If a FW device is connected to the port getting serial port data and voltages, it probably would damage the external device - not the mobo.