In the start-times 2021 I had a installation with syslinux also for testing-purpose but returned back elsewhere to above. As you have also asked about possible single-board solutions: I will answer in the corresponding thread.

Thanks for the research. It is a pity, but if syslinux works also ... well then so be it. You want to revert therefore towards syslinux?

Yes, that would be my suggestion is that for the Hyperbola ISO you guys revert to syslinux and include configs and modules for BIOS boot, 32 bit EFI, and 64 bit EFI. The installation and config is simple, and they seem to coexist nicely. This would enable installation on more machines, including the BIOS-only Thinkpads on which Libreboot can be installed. (The one I tried doesn't have libreboot yet.)

For me, I currently am using syslinux and as a workaround was able to use rsync to clone my machine to the new one, then configure syslinux for the BIOS system. There is a separate /boot/EFI directory for EFI, and /boot/syslinux for BIOS. The syslinux.cfg can be identical so just copy to each respective directory.

I haven't built an ISO before though, so grain of salt

Therefore the easiest fix would be to revert to syslinux, which supports both EFI and BIOS simultaneously.

Refind looked cool for a while, but having to run two different bootloaders isn't very KISS.

Update: Refind is strictly EFI-only, and has a .conf option that only scans for BIOS bootloaders, in other words: GRUB or syslinux.

scanfor hdbios

For now, my workaround is Clonezilla.

Actually partclone is too inflexible, so I used rsync (my settings might need some improvement):

1. Format USB stick as ext3 for file transfer.
2. Use fdisk to format and partition the new computer's /dev/sda and make boot partition active
3. mkswap and swapon on swap partition
4. mkfs.ext3 on /boot and /
5. Source machine to a USB stick [usually the install seems to be less than 6GB]:

$ doas rsync -aAhvPHX --inplace --exclude={"/home/*", "/lost+found","/mnt*/*","/dev/*","/proc/*","/sys/*","/tmp/*","/run/*","/media/*"} / /mnt/

6. Destination machine (from any live usb, I used a recent Clonezilla ISO to work around the BIOS boot problem):

$ doas rsync -aAhvPHAXx --inplace /source/ /destination/

This will copy contents of source directory to inside of destination directory.
7. Edit /boot/syslinux/syslinux.cfg to reflect kernel location, hooks and modules.
8. Run *blkid* to get the new UUIDs to copy. Update /etc/fstab with new UUIDs and /dev/sda labels
9. Making sure contents of /boot are in the boot partition, mount the boot partition and run:

extlinux -i /mnt/boot/syslinux

and

cp -r /usr/lib/syslinux/bios/* /mnt/boot/syslinux/

10. Install the MBR. Syslinux manual says it isn't needed anymore, but I had to do it as this is the only thing that worked for me. Also, the altmbr.bin did not work for me... Parabola's wiki on syslinux is the best documentation, if you follow a manual install only.

# dd bs=440 count=1 conv=notrunc if=/usr/lib/syslinux/bios/mbr.bin of=/dev/sda

11. Delete old users and add new users.
Be very careful to umount everything, taking the USB out without mounting will ruin the filesystem.