Drag and drop makes a 1:1 copy of the data files.
If you want to preserve all the errors/ error correction data/etc (like exists on copy protected disks) you will need to use something like the Blindsuite or Alcohol.
The blindsuites origins were for recovery of badly scratched discs.
Some people advise gently rubbing with toothpaste/oils/brasso/etc before reading.
It's not usually recommended unless you are grabbing everything off the disc & then turfing it though
You can use blindsuite or Alcohol to dump the DVD to an image on your HD, and then use Daemon tools to mount the image & pull the files off, if it is problematic.