The original Xbox 360 Premium configuration included a 20 GB detachable hard drive. This storage was one of the main differences between the Premium package and the launch Core System.
The hard drive stored downloaded Xbox Live content, game saves, downloadable games, videos, and other user data. Because it was detachable, Microsoft could sell replacement and larger-capacity drives as the Xbox 360’s software library expanded.
Later Xbox 360 revisions introduced larger hard drives, including 60 GB, 120 GB, and 250 GB versions. That history makes the 20 GB figure easy to confuse with the capacities of later bundles or with the original Xbox’s separate 8 GB built-in hard drive.