Windows XP like Windows 2003 has a 2tb limit with MBR volumes
Check out this KB Article which covers some details there.
When I've hit this in the past, it usually involved having to revert back on my LUN (Not too fun) and create a new LUN which is GPT.
Let me know if you have any questions on this, this is never fun to hit.. any chance for a revert?
(2TB Luns on Windows XP? - Were you also using SnapDrive, or manually expanding with diskpart?)
Thanks James, and hopefully we can get you out of this mess which the Microsoft MBR vs GPT has put you in.