Paul__B:
CrossRoads:
If external power is supplied to the 5V header pin while the barrel jack is unpowered, the 5V regulator can be reverse driven & damaged. To avoid damage, connect a diode from 5V (anode ) to Vin (cathode).That makes no sense.
For a start, the USB port does precisely that - supplies power to the 5V header pin and thereby the output of the regulator.
Yes it makes perfect sense - regulators are susceptible to damage
if reverse driven, since this reverse biases the pass transistor EB junction
(the easiest way to blow a transistor). Individual regulators have idiosyncrasies
too, especially LDO ones.
The Uno and Mega have power-switching circuits that detect Vin
and switch over to USB only if Vin is low.