Does your external HD show up in the Finder, when you do command-N and open a Finder window for example? It should do this, as long as your Finder preferences are set to let external discs show up in the sidebar. If not, you can reset the Finder prefs to do it.
Once your external HD shows up in the Finder, it really should also show up in the iTunes "Choose iTunes Library" dialog. That's the one you get when you hold down the Option (Alt) key as you open iTunes.
I suppose you've tried this already, have you?
Once your external HD shows up in the Finder, it really should also show up in the iTunes "Choose iTunes Library" dialog. That's the one you get when you hold down the Option (Alt) key as you open iTunes.
I suppose you've tried this already, have you?