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I have downloaded debian-9.3.0-amd64-DVD-3.iso from internet. I just opened it with help of File Explorer and copied its content to a usb. I tried to boot my pc with that usb, but my usb was not detected in the boot menu and I did the same while installing ubunbtu, ubuntu media was detected.(i installed it and used it for 3 months). Where did I do wrong with the Debian?