I worked for apple as a tech support agent for a year...
it is best to have all your apps. installed on your internal hd, and all your media (mp3s, projects, samples, pictures ect.) on the external. your computer can access info on the internal much faster than the external.
I can vouch for this, because I have installed osx on an external drive, and booted from it. Performance was considerably slower this way.
obviously optimum performance is desirable in this field.
my advice would be to move your itunes and iphoto libraries to the external, along with any media that you can fit.
after you move your itunes library to the external, hold option while launching itunes, then select "use existing library" then navigate to where you have the itunes library folder located.
same can be done with iphoto.
I also advise deleting any applications you dont use.
also look for a previous system folder in your macintosh hd. if you find one, delete it! it will be there if you have done an "archive and install" and it is usually a big folder that takes up alot of space.
also, put all of the sample libraries that come with your audio apps on the external as well.
your macbook will greatly appreciate this!
another thing to remember: always verify disk permissions after installing software, or software updates. Repair if necessary.
