The other variable in all this is, is that MPEG4 hours, or MPEG2 hours, which we have all learned makes a big difference.The average dvr user just wants to know how many hours can be recorded. IMHO it would be TMI to try to explain that 1.5 T of the 2 T is reserved for Dish.
Dish is marketing the drive size to impress the uninformed public.
Either way, something doesn't seem right, as 500GB/250 HD hours is only 2GB/hour, which is standard SD single-layer DVD capacity.