Hi joefromBrookline,
Thanks so much for posting in Apple Support Communities.
If we understand correctly, you are wanting to create a folder that when you click on it, it directs to a specific folder on the external hard drive.
Creating and alias folder may work for you. Give one of the steps below a try, and for more details you can go to Create and remove aliases on Mac - Apple Support.
Create an alias
On your Mac, do one of the following:
* Select the item, then choose File > Make Alias.
* You can create as many aliases for an item as you want, then drag them to other folders or to the desktop.
* Press Option-Command while you drag the original item to another folder or to the desktop to create an alias and move it in one step.
Reach back out if you have any questions.
Take care!