I have ordered an external FW800 enclosure for a spare 100GB HDD I have. I thought it would useful for installing apps onto that I don't use often thus saving some space on my MBP HDD. The question is what is the correct way to do this? Do I have to actually install OS X onto the external before I can install apps on it, as a friend suggested to me, or does my MBP recognise it as an extension?