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I'm running Windows 7 64 bit (updated all important updates yesterday) and have several external usb hard drives (self powered 3.5 inch). I recently noticed while copying files to the external drive and then using a utility to compare the original files (internal drive) to the files just copied to the external drive that the files often don't match on a byte for byte comparison. I would find that one byte has changed on the files randomly.

This is really disturbing as it happens with several external hard drives, for example a western digital and seagate and I can't rely on a proper copy to the drives.

I did a lot of research, updated windows, rand disk tests and I am still at a loss. I want to backup a lot of files to the external drives but I need to be able to trust the backup and the copy operation.

Also, I first started backing up files using 7zip directly to the external drive and that's when I noticed the errors first. Then I did a test where i compressed something to my internal drive first (verified the archive and data byte-for-byte and it passed). Then copied this archive to two separate external drives (one western digital, one seagate), then copied both back to my internal drive and both archives, when compared had files which didn't match the original versions.

When doing the above test, i used a comparison tool on the 7zip archives themselves and it recognized the archive format and showed me the files inside (thus i'm assuming it uncompressed them on the fly) and i would run the comparison several times and sometimes 3 files would not match the original versions, sometimes 1 or 2 only wouldn't match without changing the archive file itself.

This is the only time i compared archives directly so this may be a bug in the tool or errors with 7zip.

I found a support item that says 7zip has issues with creating large archives, so i'm not sure if it's related.

So after this I tried coping files directly to the external drive without compression of any sort, just through windows explorer and then comparing the files with the originals on my internal drive and they were indeed corrupted. I verified this with two different binary comparison tools.

Any help would be very very much appreciated.

New article i found describing my problem exactly: http://social.technet.microsoft.com/Forums/windows/en-US/13a7426e-1a5d-41b0-9e16-19437697f62b/windows-7-64bit-corrupting-altering-large-files-copied-to-external-ntfs-drives?forum=w7itproperf

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Try copying with TeraCopy and let us know if it's affected by this issue too. You can use TeraCopy's built in features to verify if copying was successful. I doubt that it's a 7-Zip bug, as the corruption happens when storing data on external HDD, your local archives were fine before you've copied them. –  gronostaj Dec 8 '13 at 14:15
    
Ok i'm trying it with TeraCopy right now –  Coder Dec 8 '13 at 14:53
    
All crc's matched with TeraCopy, verifying now byte-for-byte using another tool. Also the corruption happens most of the time for large sets of files but it's random. last test was for 23 files 59GB total size and in TeraCopy, the crcs matched. –  Coder Dec 8 '13 at 15:48
    
Ok that time the byte-for-byte comparison passed too, not sure what this means. It's random when the corruption happens so it might happen next time, also does terracopy change how copying is done? is it done in a different way? Does it change how copying works through windows explorer? –  Coder Dec 8 '13 at 16:23
    
TeraCopy doesn't use Windows native copying mechanism, instead it's using its own one. It also does some intelligent file queuing and other clever stuff. You can configure it to replace Explorer's default copying method if it fixes the problem. Still, it's a work around and not a real fix. –  gronostaj Dec 8 '13 at 16:38

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