> >> c) at least at first usage verify should be done after backup
> >> dd if=/dev/rcd0a bs=1m of=/dev/null progress=1
> >
> > I'd suggest doing a real verify:
> >
> > 	dd if=/dev/rcd0a bs=1m of=some_tmp_file
> > 	cmp original.iso some_tmp_file
> > 	rm some_tmp_file
> >
> > I had one disk that had 16 bytes miscompare (out
> > of an almost full 4.7 GB DVD)  IIRC it didn't have
> > any read errors, so just dd-ing it to /dev/null
> > wouldn't have caught the problem.
> 
> ins't your DVD drive broken? at least in theory correction code on DVD 
> discs should at least always detect an error.

Hmmm, if I had gotten a read error the tmp file would have been at
least a block short.  So there must have not been a read error.

I haven't studied whatever error detection code they use on
DVDs, but in general they can't detect all possible errors.

I've seen more than one error detection and correction system
fail to deliver correct data.  :-(