[psakw]

I am not a guru but just a end user so please forgive the question. After reading everything I could find including the 60 page user's guide and FAQ's I still have a question.
I bought a firewire external hard drive (500gb) for my laptop which has a 120gb harddrive in it of which 77 gb's is being used. I am presuming that I should partition the firewire drive to put my backup on it since I want to store other items on the firewire drive that are not on my internal hard drive (ie: movies and photos) . Then I presume I should have a second partition to put the 'sandbox' on the firewire drive. How big do I make each partition?
If this is the way to do the back up's. If not please tell me how to do it.