Disabling USB Supplying for an Arduino UNO

Forum 2005-2010 (read only) General Frequently-Asked Questions
8

This might do what you want:

http://www.siliconchip.com.au/cms/gallery/article.html?slideshow=0&a=110648&i=5

It is a way to supply power from your external source... with extra current... and it will shut off when USB goes away.