How to write a bootable ISO Windows installation disk to a disk partition (and not USB flash drive)?

Rufus developer here.

There's a non publicized cheat mode in Rufus that may allow you to use an external non USB drive like an external USB one, and that's Ctrl-Alt-F. Basically what this does is that it might enable Rufus to list drives that are seen as removable/hot-plug by Windows, regardless of whether they are USB based. Thus, provided that your BIOS/UEFI firmware properly tags them (you may have to fiddle with your BIOS options for that, but be mindful that not all BIOSes allow you to do that), you might be able to use an e-SATA drive.

Now, here comes the BIG DISCLAIMER: The reason why this is a non publicized option is because you will receive absolutely no support regardless of whether the option works or not and also and I make also NO GUARANTEE WHATSOEVER that it'll work for the purpose you want. For instance if Rufus happens to destroy valuable data while using Ctrl-Alt-F, you are 100% on your own. YOU HAVE BEEN WARNED.

I will also point out that this will still result in the whole drive being partitioned and reformatted, so it'll only work if you can dedicate a whole drive. If you want partition preservation then you're on your own.

After further tests, here is an (easy) working solution:

• Get a Windows10 ISO from https://www.microsoft.com/fr-fr/software-download/windows10 (it comes with MediaCreationTool.exe), the one I have is 3.23 GB

• Use 7zip to extract its content to the disk partition you want (I know it might be strange, but it works!)

• Run diskmgmt.msc and mark this partition as "Active"

• Reboot the computer and boot on this disk, it works

• Do the normal installation as usual.

I just installed a Windows 10 that way a few minutes ago.