You can easily do this with an Automator folder action. Automator is a preinstalled application for creating workflows. No programming skills required, just a bit of creative thinking. You can actually attach such workflows to folders, which are then executed whenever something is added.
Just start the application, select “Folder Action”, specify the folder in which you will put your files, drag the action “Copy Finder Items” to the right side, specify the target folder. Then you just save it with a name of your choice. Now when you add anything to that folder, this action will be started in the background and copy all the files you add to the folder on your external disk.
You can manage existing folder actions with the application “Folder Actions Setup”, which is located in /System/Library/CoreServices.