Because cross-device copying is the only place where this operation is slow, with modern HDDs all other operations are instantanious for all intents and purposes.
Good idea but you really should have put more tought on it.
going forward it seems reasonable to assume ubiquitous network connectivity, so you can simply treat external drives as a cache and show a non-modal alert "please reconnect for sync, expected to finish within X minutes" iff the amount of data synced is large enough to matter via a 3G connection.
You obviously unmount or decouple your external drive in software first and that's a good place for the operating system to inform you that:
Ongoing I/O for your USB mass storage device.
Wait for completion before unplugging.
You do? I'm often too lazy to.
You haven't met most users, have you?
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.