Can I use a portable firewire drive as a scratch disk, considering all that I have seen use 5400 rpm drives and a fw400 interface? Or should I really go for something with a 7200 rpm drive? I am not doing really intensive stuff, and most of it is either personal or 5-10 min web videos. I just want to get the video off the internal drive, since that is recommended. I also want it to be portable to go with me and my mbp when needed.