OS Installation

[Chrono]

Windows probably simply doesn't let you access the SD at all. 

Would this be the case if after flashing the SD card and ejecting it, there are, for example, PureOS-related folders visible on both, Ubuntu and Windows 10?

You could of course boot Linux from an USB stick and then flash from there.

I'll give it a try.

But it's definitely easier to use Etcher (or anything else that does the trick) with Windows directly to flash to the SD.

Haha, I did use balenaEtcher on Windows 10 to flash the SD card and the now nightmarish postmarketOS logo still showed up after booting up the phone.