Anthony:
You are awfully close. When you want to address a disk, you need only use the disk's name. For instance, I have an external HD named Claw. Here are the commands to address that disk and delete the folders which are not named Downloads:
set Claw_path to "Claw"--Make the name of the disk a variable.
tell application "Finder" to delete (every folder in disk Claw_path whose name does not contain "Downloads")--Tell the Finder to do something with the disk.
There is a short, free publication entitled "Applescript for Absolute Starters" at this address:
http://www.applescriptsourcebook.com/tips/AS4AS.html
It may help you to get on your way.
Good luck,
CAS