SOLUTION! Well, a workaround at least.
I took the Debian live USB that was created using Rufus’ “Add fixes for old BIOSes” option, and just replaced the Debian files with the Unraid files using a plain old Windows file explorer. Honestly I’m a bit surprised that worked because it really felt like I was putting together puzzle pieces that were not designed to fit together.
Here’s the recap:
- Download a Debian live ISO file: start with debian.org/CD/live, and then choose your preferred download source, architecture, and flavor, then download the corresponding .iso file. (In my case it was “debian-live-11.2.0-amd64-xfce.iso”.)
- Use Rufus to turn the downloaded Debian live .iso into a bootable USB. Make sure to check the box “Add fixes for old BIOSes”, which does some magic to make the USB bootable for my old machine.
- Download the Unraid manual install files: go to unraid.net/download and scroll down to find the .zip file for the version of your liking.
- Delete all the files on the Debian live USB and replace them with the unzipped contents of Unraid’s manual install files. There is no need to run the “make_bootable” script.
- Remember to rename the USB to “UNRAID”.