read cd's but not dvd's?

Any potential configuration-realted reasons that I can mount cd's just
fine in my dvd drive (atapi), but not data or movie dvd's (commercial
and dvd+r's)? I get an 'unrecognized or drive empty' type error. Just
a bad drive?  It makes the standard spin-up noises and some reading
sounds after loading a dvd, and it's a fairly new drive.  The faq and
man pages don't seem to suggest any configuration-realted differences
between cd & dvd.  They are both iso9660 format, correct?

Steve