From way back when... I seem to recall that disk to disk was NOT recomended because any error was normally a READ error, not a write error. Therefore a file could be made on HD, verified, then writen to CD, which at the time were several pounds each.
Therefore, if the data is important, as opposed to just music/video, 'tis the best method. I have two dvd writers onboard, but very rarley do 'on the fly' copies.