In the system partition, “disk 0” is usually your PC, so a hard drive or solid state drive in your computer.
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In the case of Windows 10, this can be done completely legally via the media creation tool from Microsoft. But it’s simpler to just download the required operating system from the internet. You can create such an ISO file yourself, though, for example with the use of tools like WinISO, UltraISO, or the burn program InfraRecorder.
Today, Windows is generally preinstalled on a system partition of a matching PC or laptop. These files used to be found on a CD- or DVD-ROM, which came with a new computer or could be acquired separately.
To obtain a high working speed (or enable the creation of a portable operating system), the selected drive should support the standard USB 3.0.
The first option seems a bit complicated at first glance, and because of the various manual steps is slightly more error-prone, but it’s still doable even if you’re not an expert. There are basically two possible methods: either manually or using automated tools.