external supply/USB programming, ground problem?

If I am running my arduino and associated breadboard circuitry from an external power supply, with the appropriate jumper setting. Can I use the USB to download sketches without disconnecting the power?

If so, do the different grounds (computer/USB vs. ext supply) conflict and cause problems?

You didn't state which model you have, but generally there is no problem using the USB port no matter what source of power is active or not.

Lefty

I have, in hand, a Diecimillia and on order a mini pro. Both of which have ext/usb jumpers The Duemillinova is the only one with auotodetect right?