Quickest way of moving a large number of files?
Solution 1:
Something like robocopy will be your best bet. USB drives can't handle a whole lot of IO to begin with.
I've pushed millions of small files to and from USB drives using robocopy. It takes time, but it gets the job done.
Solution 2:
As mrdenny said, Robocopy would be best, partly because of its robustness. The best advise I can offer, provided you're sure about the cleanliness of the files to be moved, is to make sure the antivirus software is disabled while they are being moved. You really don't want the overhead of having all those files being scanned.