Sharing my experience with the manual installation process from a linux OS distro: Ubuntu 22.04
Initially I had formatted my USB drive from the "Files" (nautilus --browser) aka file explorer gui application by right clicking the USB and choosing format "For all systems and devices (FAT)"
I double checked the format from the "Disks" utility to find W95 FAT32 (LBA) (0x0c) - I changed this to W95 FAT32 (0x0b) (As of this moment, I'm not sure if this matters.)
After extracting the download archive for 6.12.10 onto the USB, I copied the `make_bootable_linux` file to my local hard drive and continued with the installation instructions.
Here's what I found:
"unmount (not eject) USB drive" step is unnecessary. The USB is unmounted from the script `make_bootable_linux.sh` that is called from `make_bootable_linux`
the `mtools` package is required for manipulating MSDOS files (not included with Ubuntu 22.04 - `sudo apt install mtools`)
certain files located in the download archive need need execute permission - the current `make_bootable_linux` script does not handle this. I commented out lines 86,87,98, and 99 to avoid overwriting and clean up of the /tmp/UNRAID/syslinux directory. Then added execution permissions to `/tmp/UNRAID/syslinux/make_bootable_linux.sh` and `/tmp/UNRAID/syslinux/syslinux_linux`. This step could be handled upstream so these files already have execute permissions when downloading the archive or modifications made to the `make_bootable_linux` script.
also of note, the current script(s) do(es) not gracefully handle errors and appears to the user `INFO: the Unraid OS USB Flash drive is now bootable and may be ejected.` even if the above steps are not taken.
After performing these steps, `sudo bash ./make_bootable_linux` from the directory I copied this file to, now works as intended. I was able to successfully make the USB bootable and startup Unraid.