Commercail DVD has protection

You have to rip off the protection before you can burn the backup.

DVD Shrink is such a software to do this and it's free. However, usually a commercial DVD is uaually a DVD-9 disc (double layer) and most DVD+R/DVD-R discs are DVD-5 (single layer). DVD Shrink can fit one DVD-9 to one DVD-5 disc (Quality suffers too) and you have to edit out some unnecessary contents first.

Google yourself on how to use DVD Shrink.