The easiest method is to use MacTheRipper to rip the DVD, then use Roxio Popcorn to burn the resulting Video_TS folder to a blank DVD. In the handful of cases where Popcorn refuses to take the file, you can recompress it using DVD2OneX, then burn in Popcorn.
There are a handful of free, non-commercial equivalents to Popcorn but that, at least, is my method for backing up DVD movies.