reconnecting it with a diode to prevent current flow. Probably there's a good reason to not do that.
This is a hairy area. The diode is going to drop 0.7v. So 5v - 0.7v is 4.3v. And I am assuming a nominal 5v.
Ah, thanks for pointing that out.
So, the upshot is that if you're going to use an Arduino in a context where you're collecting data, and show up once a day or week or something, and plug in a laptop to retrieve the data, and you can't pull external power before doing so, you can't use the USB port for that. At least if you're using the Duemilanove or any model with the same power select logic (and haven't replaced the MOSFET). Am I understanding this correctly?
I have a Belkin powered USB hub, and now I'm wondering whether it has the same problem -- obviously there's no comm involved, but something like a bus reset if I happen to be using it unpowered, and then connect the external power.