Along with what F1 said, check the Device Manager and make sure there are no yellow exclamation marks or any other reportings indicating a related problem.

One other possibility is that a background process of some kind could be the reason, not very common but a possibility. What are the programs/process running in the background that you do not need and could close? I would suggest opening Task Manager and ending all processes that are not in use or system critical, pretty much kill all processes listed under the current user name except Explorer.exe and TaskMgr.exe, if in doubt, ask ok?

Other than these, I would try removing XP installed drivers by uninstalling the device under Device Manager, detach the drive then rebooting the system and re-connect the drive to see if it helped.

~TL