The error screen had a lot going on, including an information box that revealed the current HDD space usage. If you look carefully the dev unit was using 385gb out of 423gb. When you factor in how manufacturers rate the drives you get actual usable space of 465gb. Now subtract that from what we saw on the crash screen and we get roughly 42gb of reserved system data that is not user accessible. That number could decrease or increase on actual retail systems.