I have all the JP loops loaded onto an external drive. Not the instruments, though - just the loops. I think you can get the instruments onto an ED, I just don't know how to do it. However, there's plenty of advice on the Apple support pages.
For the loops: I'm not sure that you can load your loops onto an external drive during the initial import - the default is going to be your internal drive. However, once they are loaded, just cut and paste them onto your external drive. Then open up GB, open the loop browser, abd drag and drop them onto the browser. GB gives you the option of keeping them where they are or copying them to your local - obviously you want to keep them where they are. That's it.