First step is to use automotive polish to remove the deep scratches on the DVD. Be patient. Could take hours. I use Meguiar's Deep Crystal Deep Gloss Polish and bathroom tissue paper. Change paper often as you polish out the defects. Use firm pressure to remove deep scratches and follow up with medium pressure/clean paper to achieve and smooth finish.
Reduce the read speed when copying the files back to the hard drive. Download AnyDVD and go into the SETTINGS option to select SLOW/QUIET MODE.
I haven't lost a single disc yet. Note that if the dye in the DVD has degraded, then you may only be able to transfer those unaffected chapters to the hard drive and re-assemble them using Shrink (FREE), Nero Recode, or similar applications.