Hard to say...
I suspect the best solution would be to get an external HDD enclosure, and put a high end HDD in it.. desktop USB drives tend to come with the cheapest model that will provide X GB.
Generally, though the bottleneck is more to do with the USB 2.0 specification than the HDD itself.
I'd recommend an eSata card or a fibre channel card and external storage/ HD caddy to match if it's at all possible to add it to your kit. Even firewire 800 would be better. Sounds like you need a better port than than usb for the job.
Esata is probably the cheapest option, cable, card and caddy will come to about £50, then put whatever drive you want in there.