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