From a story at Wired News.
Several of the new, more robust high-definition DVRs will sport 250-GB hard drives — enough room to store 30 hours of high-definition programming or about 200 hours of standard broadcast.
The beefier hard drives are necessary because signals for high-definition TV, or HDTV, carry far more information per frame than standard TV. For example, one HDTV standard in the United States (1080i) calls for images that are 1,920 pixels by 1,080 pixels, refreshed 60 times a second. By comparison, standard analog TV in the United States is 500 dots by 525 dots.