Windows XP has a maximum partition size of 2 TB, which is a real pain for me since I have a 16-drive NAS. At the moment I just access it in 2 TB chunks, but as I add drives this will get to be even more of a pain than it is at the moment.
So I'll need to upgrade the operating system that creates the data which goes to the NAS to something that supports large partitions. With an eye to the future, ideally I'd be prepared for up to 4 TB * (16 - 2) = 56 TB with RAID 6, though for the short term even 6 TB would be an improvement. (Maybe 16 - 3 would be better, leaving room for a hot spare?)
I've heard that Windows Server 2003 SP1 has a large limit, but I'd like to avoid it here if possible.