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.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.
ddrescue does not write zeros to the output when it finds bad sectors in the input, and 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 without wiping out the data already rescued.
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 errorfree 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.
recordable cd and dvd media keep their data only for a finite time (typically for many years). after that time, data loss develops slowly with read errors growing from the outer media region towards the inside. just make two (or more) copies of every important cd/dvd you burn so that you can later recover them with ddrescue.
the logfile is periodically saved to disc. so in case of a crash you can resume the rescue with little recopying.
ddrescue also features a "fill mode" able to selectively overwrite parts of the output file, which has a number of interesting uses like wiping data, marking bad areas or even, in some cases, "repair" damaged sectors.
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