Just got a new external DVD burner for netbook and it works just fine. However, I have burnt DVDs from 3-4 years back and they are being read as blank DVDs and won't play. What's weird is these burnt DVDs only ever worked on the laptop they were burned on and 1 other set top DVD player I now do not own. They don't work in my new DVD set top player nor either of my desktops (And obviously this new external DVD burner). Why are they being read as blank?