This behavior can originate from two possible sources.
The most common problem is a malformed DVD. The DVD Player program depends on a specific disc structure present in DVDs. If commercial DVDs play as expected, but home-made DVDs do not, recreate the DVD using iDVD, or Toast’s DVD Movie function
The other cause is the version of Apple’s DVD player. Older versions of DVD Player only worked on internal DVD drives; not USB or FireWire devices. Update Mac OS to use the DVD player on an external drive.