I don't know if you've already sorted the problem, but in the following article on dual booting, it says in "Downgrade situation 2" on page 6 that the worst case is a laptop with a SATA HDD, where XP requires drivers to be loaded from the laptop's floppy drive and confirms that a USB floppy will not work.

http://www.pcstats.com/articleview.cfm?articleID=1910

Unfortunately it does not provide your answer, but it is a useful article.

Best of luck. Eddie_