I am thinking about getting a portable backup drive when I am out on vacation or shooting a lot of pictures when traveling. Assuming I am shooting all of the pictures in JPEG + RAW (since that will eat up the max amount of file space) what size drive do you think I should go with? I will more than likely have my MBP with me. Using Aperture 3 I would upload them to my MBP but I would want to do a separate back up as well. So 320GB? 500GB? Higher? I am thinking of using this in the field so to speak since I have Time machine and a 500GB Western drive at home. Any thoughts much appreciated.