In Windows, it's just that a MBR
(default) formatted disk only supports up to 2TB
per single partition. While a GPT
formatted disk supports up to 256TB
per single partition. You don't want to have your Windows disk formatted as GPT since it render Windows unbootable.
Looks like it's only up to 32 GB
for Xbox, but you could just create a 32GB partition for it to use. USB Storage Device Support for Xbox 360 - Xbox.com
Hope this helps,