Trigger off rsync by just plugging in a USB drive?
You are probably going to want to look into a Udev rule to launch a script. What's good about udev rules is you can lock down what usb drive is used for backup so not every usb drive fires it off.
First step is to get your udev rule going:
Here is a great tutorial on writing udev rules
and here is another
Then you can move on to your scripting aspect. The first link covers a bit of launching the script from udev so then you just need to call rsync from it.
man:rsync has a lot of information on usage.