Disable USB power so when connected via USB without Ext power, Arduino is off

sonnyyu:
You CANNOT just assume since your USB device is plugged into a power source that you do not need the power lead on your USB cable! That is just plain wrong.

Yes, you're right - I wasn't planning on doing that and am aware the ground would be required. I would rather not rely on a custom USB cable though.

LarryD:

never from USB

Remove the USB polyfuse.

That's just what I was hoping for, could you describe where that is on the board so I know what to remove?

Thanks for your help.