I am looking to acquire an External HD for backing up my iMac and also for storing, accessing and playing videos, songs, etc. I think I'm probably in the range for a 250GB one with both Firewire (400) and USB 2.0. Does anyone have any recommendations for the best brand and seller? As I will primarily be using this for video, does anyone know how an External HD compares to the internal HD for response time and usability with apps like iMovie, etc.?