Detecting a mounted volume

I'm sure this has been asked and answered, but I can't seem to find it on the forums.
How do you make Hazel detect when a volume is mounted (such as an external hard drive)? I have a bunch of rules for moving files off to external storage, but only want them ran when the drive is connected.
Any help would be appreciated.
Ron
How do you make Hazel detect when a volume is mounted (such as an external hard drive)? I have a bunch of rules for moving files off to external storage, but only want them ran when the drive is connected.
Any help would be appreciated.
Ron