Most drives in the past (~1980s through early 2010s) had 512-byte sectors. Recently they've moved up to 4096-byte sectors. How does this help? Because each sector needs error correction codes, and it's more efficient to store 8 sectors' worth of information and error-code it once than to do it 8 times separately. This translates to higher drive capacity. The only catch is that ordinary BIOS-based computers can't boot from them (but they can be read as data drives by a running OS).
Most people argue that Windows XP can't read drives over 2.2 TB because the MBR partition table can only address 2^32 sectors, and since sectors are 512 bytes this means a 2TiB limit. One answer is to use GPT, but XP can't read that, so they conclude that GPT is the only option and hence XP is barred from using big drives. Microsoft's official stance is that XP can't read big drives, although I think this is to encourage sales of their newer operating systems.
The 2^32 part is right, but what these people fail to see is that MBR is addressing sectors, which is an arbitrary measure. Since drives now are 4K, each individual sector holds more, so bigger drives can be used by old operating systems.
This is all well and good if you buy a drive, because it would be pre-formatted. But what if you want to format the drive yourself? You shouldn't do it in an older operating system (probably includes Windows Vista) because, although it will format and be valid, the OS doesn't know to align the partition based on the sector size. Aligning it to an arbitrary offset was fine for 512-byte sectors, but it would slightly reduce performance on a 4K drive. Your partition should be offset by a multiple of 4096. If you want to verify that it is, then (under Windows XP), Click Start, Run, type msinfo32 and press Enter. In the System Information window that pops up, go to Components->Storage->Disks. Find your 4K drive and look for Partition Starting Offset.
One more concern voiced on a forum was that XP would only read the first 2TiB and experience errors on anything higher. I have disproved that by filling my 8 TB drive with about 3 TB of data and doing a search in Windows XP. There were no errors, so I am confident that XP had no issues. Besides, I went through some of the directory structure myself and looked at a few photos from a huge family album that has to have crossed the 2TB mark.
I think I've said enough. Here's a screenshot of the drive in Windows XP.
In a nutshell:
Myth #1: You can't use >2TB drives on Windows XP because they require GPT
Truth: While it's true XP doesn't support GPT, large drives don't need it*, so you're fine.
*Of course, if your >2TB drive has 512-byte sectors then you are out of luck.
Myth #2: XP only supports 512-byte sectors; Microsoft's official stance
Truth: Yeah, right. See first screenshot, in the Bytes/Sector field.
Myth #3: XP will only read and write the first 2 TB and then fail
Truth: No, my full-drive search under XP proves this is false.
I hope this helps the many uninformed people (like myself, previously) who waver over a drive purchase. I have backed up my points with real tests and pictures, not the unconfirmed advice you find on forums.
It's quite refreshing having recently made the switch back to XP. Its clean, simple interface is unparalleled and the speed of Windows Explorer is amazing, even in the face of huge amounts of data. It's no secret why Windows XP still has nearly 11% market share. A new computer should be coming in the mail soon, and it has Windows 7 x64. I'll use that for things that need modern Windows.