Copy on the fly? Very small print on an insect? The writing on the zipper of one's trousers?
Making a copy of an optical disc from one drive to another without the intermediate step of ripping to a hard drive first. Can be done with unencrypted discs with no issues. And even with encrypted video, if you use a program like AnyDVD HD in the background, it is possible to copy straight over to another disc of the appropriate size if you are copying without compression. But I have always advised ripping encrypted video to the hard drive first. It may be an extra step, but there are fewer variables to go wrong.