FW 800 isn’t 800 MegaBytes per second, it’s 800 Mbits or about 75 MBytes per second. The ATA 133 hard drive can run as fast as 133 MBytes per second. So, I guess according to specs the internal hard drive isn’t the problem. If I know anything about Final Cut Pro, you should be running an internal drive as your scratch drive. I doubt any external will work, unless it’s a thunderbolt drive.