TrueNAS Core Vs Scale - Grub issue

I was using Core for a bunch of time now without real issue on my HP DL380 G5.
I upgraded all my other machines without real issues except this server.

Symptoms:
I install a fresh Scale on 2 USB external mechanical hard drives, installation goes smoothly (BIOS only, not UEFI), I reboot, I set the truenas_admin password, log in and everything seems OK. As far as I restart the machine, GRUB reports an error (ex: unknown filesystem) and stays in rescue mode

Tests:

  • tried to install from version 24.04 to 25.10.1, the grub error can change (attempt to read outside disk, “grub grub grub”, unknown filesystem, and maybe some other one I do not remember)
  • tried to install on a single disk
  • tried to boot systemrescue+zfs, the boot disks seems ok (as far as I can tell)
  • tried to scrub the boot-pool after the first successful boot and reinstall grub on both hard drives
  • tried to change the 2 hard drives and tries to change the USB/SATA adapters
  • tried installation from DVD, from USB stick created with rufus (latest version did not asked me DD mode), from a Ventoy different USB stick which loads the ISO
  • tried to reboot without changing anything and tried to reboot after reloading backup configuration

As far as I saw, the boot loader is different from Core and Scale which can explain why only Scale has the issue.
What is really surprising is that the first boot succeed, then, I cannot boot anymore without reinstalling.

Note: I might be slow to answer because I will have a very limited access to the server for the next 2 weeks.

Any advice is welcome. Thanks

Please strictly follow the official recommended upgrade path.

I forgot one of the tries:

  • I upgrade using the web interface from latest 13 version