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. 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. It should be very fast, in the case it's a SSD for example. A nice solution would be: put the ISO installer on Partition 2 of the actual disk where it's going to be installed, then boot on this Partition 2, and install Windows on Partition 1. Why to do this? Here's the reason: I've installed Windows many many times, but at the end it's always slow from a USB flash drive. Partition 1 of the same disk is where Windows will be eventually installed once it will be installed, I will be able to delete this no-longer needed Partition 2.

If possible, I'd like to install the bootable Windows 10 installer ISO into (bootable) Partition 2 of a disk. The latter is important because the main solutions of How do I place a bootable ISO on a USB drive? don't work: Rufus, for example, doesn't offer the possibility to do it on a hard drive partition (I just tried now, the 'Device' list is empty when I insert a USB flash drive, here it works) More generally, how to write a bootable ISO to a disk partition with Windows? I'd like to put the Windows 10 (or 7) bootable ISO installation disk into an external disk partition (and not into a USB flash drive). I have already read How to write bootable iso to a partition? and How do I place a bootable ISO on a USB drive?, but here it's a bit different:

Windows 10 iso on usb