Safe/proper ways of connecting power to RAMPS/Mega? (2 replies)

Hi,

I have in my posession Ramps 1.4, Mega clone rev3 with small LCD and SDcard and Xbox PSU 12V 16A. I would like to connect it all and it's pretty straight forward when it comes to connecting power to RAMPS.
My question is what next?
How do I power up Mega if I'd like to use it without PCs USB port? I understand that D1 diode on Ramps should be soldered on if I want to power up Mega from RAMPS. But what would happen if I connect USB at the same time? Won't I fry anything? Should I splice open USB cable and cut 5V lead?

Let's imagine a situation:
I have fully portable 3D printer which is powered by XBOX brick (D1 is soldered in) and SDcard-print capable. From time to time I need to hook it up to a computer to print directly from it. I don't want to disconnect power from arduino just to plug in 5V USB. I'd rather hot-plug USB cable without worrying I'm gonna fry anything.

Sorry for asking such simple questions but everywhere it's written about hooking up RAMPS or Mega on it's own and nowhere on how it works actually. I just don't want to fry the electronics.

Thanks for your help.
-m