Seeing as the Xbox 360 ran an HD-DVD off firewire, I'd believe it would be fast enough on PC as well. However, I'd still find alternatives.
There are some cards that allow for external SATA drives;
http://www.newegg.com/Product/Produc...82E16839200006
(PC cards are available too, depending on your machine)
You could probably invest in an internal laptop drive as well. I didn't think they were tremendously expensive, but still harder to come by.