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The most common cause is that the drive can't read your media. Knowing nothing about your DVD or where you got it, anything else would be a wild guess of little use.
 
do you watch it in the same player?

if so, my guess would have to be that if it doesn't play, its because your
DVD player is older than the DVD format its written in.

For example, my parents DVD player is one of the originals, and it only plays bought DVDs. But if i had copied a movie on to a DVD+R disc, and tried to play it in their player, it will not play. This is because the DVD+R format is a relatively newer format [newer in comparison to DVD-R].
 
I was watching kingdom of heaven directors cut. RETAIL! Yeah, it will just freak out sometimes and not read the disc.. Just wondering if you may have idea to solve this or anyone experienced it.
 
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