What happens if a root partition, mounted over USB, disconnects?

I have a laptop without an internal hard drive. To get some use out of it, I'm considering putting the root partition on external media, over USB. I can't see any reason this wouldn't work, but I wonder what would happen if I accidentally disconnected and reconnected the USB. Obviously in general it's possible to reconnect devices automatically, but if the root partition disconnects, would this still be possible? I am pretty sure that a running system would not immediately die if the root were to disappear, but I would not be surprised if remounting became impossible.