Why does copying a DVD stop halfway?

When I attempt to copy a DVD, I go to Disc Utility, Select the DVD I want to duplicate which is in the D drive, Select New Image, put in a name to Save As, select what file to save in (Documents), select compressed and no encryption, and start the save process. All seems well until about half through the copy process when it stops and I get the error message "Unable to create "whatever-1.dmg" (input/output error). I've tried this on 3 seperate DVD's and get the same result. The DVD's I want to copy are a copies given to me from someone else and are not copy write protected. Is there something I'm not doing correctly such as the file I'm saving them to or whatever? Thanks,,,,Ron