You can play HD content, just not HD-DVD or Blu-Ray media. HD is just describing a resolution; take 1080p for example, which is 1080 pixels high by 1920 pixels wide. The data disc formats HD-DVD and blu-ray are just larger optical mediums using different technologies to achieve the goal of holding all the information required for HD content (movies, games, etc…). And yes the superdrives in both your macs should write dual layer discs.