Installer iso-files that couldn't be put on a usb-stick doesn't seem to
make sense. The file is called Win10_2004_English_x64.iso
After the iso-file is on the stick Gnome Disks gives the contents as
UDF version 1.02 and it can be mounted.
A common mistake is to just copy the .iso file to a USB key. You want
the contents of the USB key to look like the contents of the iso file.
I have to see if I can find any special instructions from
microsoft.com, although they would probably not cover how to do it on
Fedora
It should not matter what OS is on the system disk when booting the Windows
installer (what the Windows installer does to Fedora may be an issue).
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