If you want partition preservation then you're on your own. 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. For instance if Rufus happens to destroy valuable data while using Ctrl- Alt- F, you are 100% on your own. 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. 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. 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 burn iso to usb