I think there is an important distinction to make here.
The original poster said he wanted to make a COPY of an existing DVD.
This is opposed to RIPPING the contents to disk (for viewing from the computer).
I, too, have run into difficulties in trying to copy DVD's.
In my case, a friend had some home videos burned to DVD from a non-digital (tape) camcorder. The DVD's had been created by a "standalone" DVD player/burner (that is, they had not been processed via computer first).
These things are a PAIN to dupe. Not like duping a music or data CD.
Even without commercial copy protection, I could not use a program like Toast to simply "copy" it -- kept getting errors.
Is it correct to assume that a video content DVD must be RIPPED to the computer first, and then re-COPIED to a blank disc?
There have been numerous threads about which DVD copying/ripping software to use, but what I'd like to see is a modest tutorial explaining _how_ a DVD is constructed (file-wise), and the proper steps required to dupe it using any available program.