can't cp files from dvd

[ric@otte.ucsc.edu]

Out of curiosity I thought I'd try to copy files from a movie DVD to my
hard drive.  When I tried to copy some of the .vob files, I got a
message that said something like input/output error, and nothing would
copy.  This isn't real important to me, and I'm mostly simply curious,
but is it possible to copy files using cp from a mounted dvd?  I had
thought they should be able to be copied, but now wonder if there is
some sort of hardware block that prevents this (my dvd reader is old) or
if there was some trick I didn't know.  Thanks,

Ric