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10 Generate mode

NOTE: When ddrescue is invoked with the ‘--generate-mode’ option it operates in "generate mode", which is different from the default "rescue mode". That is, if you use the ‘--generate-mode’ option, ddrescue does not rescue anything. It only tries to generate a logfile for later use.

So you didn’t read the tutorial and started ddrescue without a logfile. Now, two days later, your computer crashed and you can’t know how much data ddrescue managed to save. And even worse, you can’t resume the rescue; you have to restart it from the very beginning.

Or maybe you started copying a drive with dd conv=noerror,sync and are now in the same situation described above. In this case, note that you can’t use a copy made by dd unless it was invoked with the ‘sync’ conversion argument.

Don’t despair (yet). Ddrescue can in some cases generate an approximate logfile, from the input file and the (partial) copy, that is almost as good as an exact logfile. It makes this by simply assuming that sectors containing all zeros were not rescued.

However, if the destination of the copy was a drive or a partition, (or an existing regular file and truncation was not requested), most probably you will need to restart ddrescue from the very beginning. (This time with a logfile, of course). The reason is that old data may be present in the drive that have not been overwritten yet, and may be thus non-tried but non-zero.

For example, if you first tried one of these commands:

ddrescue infile outfile
dd if=infile of=outfile conv=noerror,sync

then you can generate an approximate logfile with this command:

ddrescue --generate-mode infile outfile logfile

Note that you must keep the original offset between ‘--input-position’ and ‘--output-position’ of the original rescue run.

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