Running rules only when a volume is mounted?

I just downloaded the Hazel trial. Very impressed so far. I had a specific use case
which is to move all files in a set of folders more than 4 years old to the same folder structure on a separate, removable drive. I did this with rules to iterate through the sub folders and move the required files retaining the folder hierarchy (based on examples I found online).

But how do I run these rules just when the removable drive is mounted? It obviously doesn't make sense for Hazel to try to move the files when it is not.

For now, I have paused the folder and disabled the rule until I figure how how to add an extra condition to only run when the removable drive is mounted.

Thanks for any advice or sample rule.

You can just set the destination folder. If the drive is unmounted, the rule will fail but Hazel will retry the rule until it succeeds (which would be when it's mounted again).