Windows XP (SP2) can use dynamic volumes that are over 2TB:
"Because partition tables on master boot record (MBR) disks only support partition sizes up to 2 terabytes, you must use dynamic volumes to create NTFS volumes over 2 terabytes. Windows XP Professional manages dynamic volumes in a special database instead of in the partition table, so dynamic volumes are not subject to the 2-terabyte physical limit imposed by the partition table. Therefore, dynamic NTFS volumes can be as large as the maximum volume size supported by NTFS."
found here
Therefore, you have to use dynamic volumes in the disk manager (right-click on My computer, select "manage" and then find the "disk manager") in Windows XP (I don't think it is necessary to use dynamic volumes on the open-e side, just the regular "volume group" etc. stuff) if you want to use volumes over 2TB. Also, i think that the default cluster size is 4K, so there's another limit at 16TB usually.