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You want to do a so called "on the fly" copy? The only way you can do this is with unprotected DVDs. If you want to copy a movie (almost all commercial movies) the anti-copy protection don't allow you to do this.

Moreover, if the source disc is a dual layer you must first compress the source video to fill on a single layer disc. And the entire process require much time.

The faster way to do this is rip the original on hard disc, elaborate it, and then burn on a different disc.