making backup copies of damaged dvds using ddrescue
I would normally use AnyDVD (on Windows) to take a backup of a DVD with copy protection, but is doesn’t seem to handle read errors very well (e.g. discs have small scratches) and copies fail.
GNU ddrescue (gddrescue, ddrescue, NOT dd_rescue) on linux is supposed to be better at handling small errors and is designed to handle read errors.
So far I have found that the best way is to take an quick(ish) initial copy;
ddrescue -b 2048 -n -v /dev/sr0 image.iso image.log
This reads the whole disk once, marking any bad blocks as “non-trimmed”. It then reads again in the other direction retrying any bad blocks. Any blocks that still fail are “trimmed” then marked as non-split. There is no further processing after this because of the -n option.
If there are any read errors do another pass using the direct option (-d). You must also specify the number of retries that bad sectors get (-r), otherwise they will be ignored;
ddrescue -b 2048 -d -r 3 -v /dev/sr0 image.iso image.log
… if there are further errors then you can use the -R (retrim) option to retry full sectors (taken from Forensics Wiki). This will try the any bad sectors in a different order which might help read some (according to the ddrescue documentation, see link below);
ddrescue -b 2048 -d -r 3 -R -v /dev/sr0 image.iso image.log
I have yet to investigate using different drives to read the disc for another pass, but this might also help.
I don’t think this method (using ddrescue) removes any copy protection.
Full manual for GNU ddrescue; http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html
Reading discs might take a long time; http://old.nabble.com/10-days-for-7.8gig–td21461792.html
Good documentation about Data Recovery and ddrescue; https://help.ubuntu.com/community/DataRecovery#Data%20Recovery%20from%20damaged%20filesystem%20or%20drive