Why let the exception case (removing an external drive quickly) ruin the main use case?
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.
Because it's not really the exception. I rarely copy or move files except when I want to transfer them to a USB key. Otherwise,they mostly stay where I created or initially downloaded them to.
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.