Installing an app to run from an external HD
If the app doesn't let you configure where to store files, you can symlink the folder to your desired location.


Move the folder where the data is stored to the new location.
Run the following to symlink the folder:

ln -s /Volumes/externalname/folder ~/Library/path/to/folder



You can move the application by itself, but that won't change where the app stores data files.