Trigger Automator workflow on HDD mount
I just popped open Automator to see if any actions are available that could be strung together to do this. It looks to me like this is something that you would need to create using Apple Script.

Take a look at the this forum thread:
http://hintsforums.macworld.com/showthread.php?t=115494

The post by rccharles has example apple script that could be used to accomplish this. The other posts above this one also have some alternate suggestions that could do what you're looking for.