I drag and drop iMovies to drive. They copy ok (takes forever). when I try to play the movie, I can see all of the clips,but the audio and video don't play.
When I saw that, I was going to diagnose your problem right away: the drive's fine, the OS is fine, but you, my friend, are on USB 1.1 at best. Of course, I read your subsequent posts and you've already discovered that. kudos!
I do want to say that yes, you should definitely get the firewire drive. I know alot of people will use a USB 2.0 drive and find it satisfactory, but I don't- ever, unless it's just for documents or something. but even tho USB 2.0 is faster than FW400, it has a variable speed rate, so for video, you never know what you're going to get.
Also, those Porsche drives are great- I've got 2 of them and I use them all the time, but I don't use them when I EDIT video. They don't have a fan, so after extended intensive use they turn extra-toasty, and you can't cool them down. If you REALLY use them hard and long, they could even damage themselves, all because porsche didn't want to add a fan. I think for the use you described, it'll be perfectly sufficient. Much more than that [for instance, if you want to put video onto the drive, then use those files to edit movies in imovie] I'd start to reconsider the drive entirely and get a lacie D2 or Big Disk Extreme... [Firewire, of course

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Good luck