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I am trying to recover data from damaged hard disk that sometimes outputs corrupted data – I copied all files from it twice, compared the files and some bytes in sone files where different.

Is there any tool that copes with such disks, reads them several times, compares data, or reads unverified sectors again and outputs (as much as possible) non-corrupted image of the disk?

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DDRescue does exactly that, I've used it many times to recover data from hard disks and even old, scratched CDs. It is for example contained on the SystemRescueCd (also bootable from USB). But the problem remains if the hard disk output wrong data without reporting it as an error (which it really shouldn't) - how many times do you need to read a sector until you are reasonably sure that you've read the correct data? I don't think this can be solved in a simple copy program, since you would need to validate the correctness of the data which a dumb copy process can never do.

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I think drive electronics may be damaged, so I suppose when reading entire drive twice and afterwards unverified sectors once more would be sufficient to recover sectors that can be easily recovered. Is this possible using ddrescue? – DavisNT Sep 18 '12 at 9:20
Only when the drive reports the error. If it just reads the data fine but returns bogus data, I don't see how any software could detect this. – Stefan Seidel Sep 18 '12 at 9:51
By reading disk twice and comparing both reads. – DavisNT Sep 18 '12 at 9:54
Which one is the correct one then? What if the disk returns bogus data twice, but differently? – Stefan Seidel Sep 18 '12 at 10:04
Then probability that those disk can be correctly read without specific hardware tools is relatively small. ;) I suspect that disk has problems in electronics part and CRCs from disk surface are checked and data is crippled between disk electronics and IDE controller. – DavisNT Sep 18 '12 at 13:16

You might want to try GRC.COM's Spinrite. Supposedly it has a mode of recovery where it will read a sector, check SMART, check the CRC, re-read if necessary and just rinse/repeat while compiling a statistically based table of the most likely "correct" data. Check their website for details.

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There are commercial programs. The one I used did the job for me, but of course there are no guarantees. If the data is really important, perhaps you should consider let the professionals handle it. The disk does not get any better, and it might be getting worse when you try.

Oh, you don't want to hear this, but you need to. Where are the backups?

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What is the name of that commercial program? – DavisNT Sep 18 '12 at 7:15
Recovers bad sectors so you can copy data off of the – Moab Sep 18 '12 at 15:12
I used Ontrack Easyrecovery. – fstx Sep 18 '12 at 18:19

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