I have a Windows 7 Installer (x64, ultimate, SP1, u) on my NAS. I checked its SHA-1, and it was correct (starting with 36ae60de). I copied this .iso file to my pendrive (SanDisk Extreme with very high writing speed) because I wanted to customize it on my other computer (that one does not have access to the NAS, yet; but has free space on its SSD), and copied it to its Desktop.

I checked the SHA-1 of the copy on the desktop, and it was different, therefore I checked the copy on the pendrive, and it had the same wrong SHA-1.

So I repeated the whole copying multiple times. Sometimes the hash was correct, sometimes it was not. I also checked my pendrive for bad sectors but nothing was found.

Should I suspect that I have some kind of malware that can identify Windows 7 installer .iso files and infect them? Or is it just a hardware error in the copying? In the latter case what is more probable: a defect in the pendrive or a network fault (my computer and the NAS are attached to the same router with a 1-2 meters of UTP cat6 cables)?


I used CertUtil -hashfile x.iso SHA1 from a command line to generate the hash. I compared it to the hash found here: https://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/windows-7-service-pack-1-digital-river-downloads/4987a673-4e1c-44f1-8c40-4be30405d07f

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    What are you using to generate the SHA1 hash? How did you determine what the SHA1 hash is suppose to be? Be specific and edit your question – Ramhound Dec 8 '17 at 11:52
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    Were incorrect hashes the same? or different each time? – Kamil Maciorowski Dec 8 '17 at 12:01
  • Some users have reported errors copying large files to USB drives on Windows 7. Try copying the file with TeraCopy as a workaround. – djones Dec 8 '17 at 12:29
  • @KamilMaciorowski: They were different. – z32a7ul Dec 8 '17 at 12:59
  • @djones I am interested in the details (how often does it happen, what factors influence it, how to detect such error, is it true for win7 only, etc.), can you give me a link? – z32a7ul Dec 8 '17 at 13:03

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