Description:

ddrescue copies data from one file or block device (hard disk, cdrom,
etc) to another, trying hard to rescue data in case of read errors.

ddrescue does not truncate the output file if not asked to. So,
everytime you run it on the same output file, it tries to fill in the
gaps.

If you have two or more copies of a damaged file, cdrom, etc, and run
ddrescue on all of them, one at a time, with the same output file, you
will probably obtain a complete and error-free file. This is so
because the probability of having damaged areas at the same places on
different input files is very low.

If you also use the bad blocks file feature of ddrescue, the data will
be rescued very efficiently. Also you can interrupt the rescue at any
time and resume it later.