High time you told us how and what you are powering.
I thought I had expressed the objective earlier. Apparently not to everyone's satisfaction.
I'm powering a number of controls, relays, contactors, and such to run a heating element, pumps, and valves up to 50 amps at 240 volts. Sure, the Arduino is not powering that but it is, through relays and such, controlling them. The USB power may be enough to drive all of that but from several sources I find that an external power supply roughly doubles the available amperage which is a good fudge factor to have. The unit will be powered that way once complete anyway.
My main objective is, when I turn the main switch OFF on the unit, I don't want power from the USB keeping those switches closed and allowing the high voltage components to stay live. It's a safety matter. Normally this won't be an issue as there will be no USB connection during normal operation. Only during development. The unintended live voltage is even more of a concern at that point as not everything is packed away nicely in the control panel.
Now, back to the point at hand.
I added a USB connector to a proto-shield as I started the actual build. From there, I connected all but the +5v wires to a USB jumper to the Arduino. Now when I kill the power, everything goes dead. Mission accomplished on that front. The down side is, when I flip the power back on, the board powers back up but it DOES NOT re-establish the serial connection over the USB port. I have tried the reset button, recompiling, closing and re-opening the Arduino software, but nothing works. From the "Tools" dropdown, no port is listed. The only solution I have found is to disconnect, the re-connect the USB cable. Do that and all works fine again.
Less than ideal but at least shutting off the switch reduces the possibility of electrocuting myself. If anyone has any ideas that might help with re-establishing the serial comunication without messing with the cable, please let me know.