There's an article in the most recent PCA which covers USB devices and, I think, references a utility that might help you. Sorry I can't be more specific but I don't have the mag. in front of me. I just remember seeing something.
HI, YOU CAN TEST HDD SPEED WITH "HDTACH" IM SURE ITS FREE DOWNLOAD OFF GOOGLE ETC. YOU WILL NOTICE A MARKED DROP IN SPEED COMPARED TO YOU ONBOARD HARD DRIVES, MY EXTERNAL RUNS MID 20S MEGA BITES PER SECOND WHEREAS IDE HDD GO MORE THAN DOUBLE THAT AND SATA EVEN MORE. ALSO AS NOTE I HAVE FOUND CONECTING CASE MOUNTED FRON USB TO SOME MOBOS CAN RESULT IN THEY AND OTHER USB SLOWING DOWN TO 1.1, TRY DISCONECTING CASE ONES IF YOU SUSPECT THIS IS A PROB