I have not heard of anything like this outside of Apples Time Machine in OS X. Even then, it creates a system image and it only backs up new/updated files for each progressive backup. As far as scheduling it to backup when an external is plugged in, I have not seen any programs like that.
A few minutes of searching on Google turned up this:
http://www.genie9.com/home/Genie_Tim.../overview.aspx
I have NOT tried or tested it, but from what others have said, it works similar to Apples Time Machine.
You can give it a try, but again, it will most likely want to create a system image for a starting point for backups.
-JMP