Using iMovie with external USB 2.0 drive

I use iMovie on a 2-year old MacBook Pro. The MacBok Pro has Firewire 800 and but only USB 2.0. I've been using an external Firewire drive with it for storing my movies. Now it is full and I need to get another external drive. However all the new drives I've seen are only USB.

My question: Will it be a problem to use iMovie using USB 2.0? (The new drives are all USB 3.0, but the computer is only 2.0). Will I be able to edit movies that are on the extenal drive? Or will I need to store the "in-progress" movies on the computer, and then move them to the external drive when I'm done with them.

And when I ask "will I be able to" I mean doing it without seeing endless beachballs...

Thanks!

Dan