Actions triggered by plugging an external drive.

Let's say I have photos on my computer that I want to copy to an external drive, I do this manually, but I was just thinking? Maybe I could create a rule for that... but I couldn't figure it out, how to run a rule only if a specific device is plugged... would be nice isn't it?

Is the device in question the source or destination drives? If not, then what is the relation?