But there's a whole class of problems here, not just removing an external drive. I'm at my friends house, copying files from a network share over his wifi. I'

But there's a whole class of problems here, not just removing an external drive. I'm at my friends house, copying files from a network share over his wifi. I'm copying from mounted network shares at home over the internet. I'm putting files on, or pulling them off, a USB stick. If copy turned into the feature you described, there would have to be a 2nd "no, REALLY, copy" command. Hardly a step forward. That being said, there's no reason it couldn't work the way you describe if the system knows it's copying to/from local physical discs.