This may or may not address your problem. I just upgraded from XP to a new PC running Vista. MY external USB 2.0 hard drive used to work quickly
(30 MB/sec), but now was running extremely slowly (1.5 MB/s, if I'm lucky).
Vista would tell me that my device could perform faster if I connected it to a USB 2.0 port. Well guess what? All my computer has are USB 2.0 ports. Other ports were no help.
I tried all sorts of fixes, nothing was working. The hard drive would still work fast on my old PC.
One forum suggested that some USB cables don't fully support USB 2.0.
So I tried a different USB cable, and it works perfectly. 25.6 MB/s! Put the old cable on, and it was slow again, with the same error message. Put the new cable on, and it worked fast again.
So consider trying a new cable!