GNU ddrescue is a data recovery tool. It copies data from one file or
block device (hard disc, 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, every time you run it on the same output file, it
tries to fill in the gaps. The basic operation of ddrescue is fully
automatic. That is, you don't have to wait for an error, stop the
program, read the log, run it in reverse mode, etc. If you use the
logfile feature of ddrescue, the data is rescued very efficiently (only
the needed blocks are read). Also you can interrupt the rescue at any
time and resume it later at the same point.

Automatic merging of backups: If you have two or more damaged copies of
a 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. Using
the logfile, only the needed blocks are read from the second and
successive copies.

WWW: http://www.gnu.org/software/ddrescue/ddrescue.html
Author: Antonio Diaz Diaz <ant_diaz@teleline.es>