Uefi

On a UEFI Ubuntu system there’s a 123-byte file on the ESP that almost nobody looks at. Mine was at \EFI\ubuntu\grub.cfg and contained this: search.fs_uuid 0a9ccc8b-d14c-4291-9eb8-7bf71995a901 root hd2,msdos1 set prefix=($root)'/grub' configfile $prefix/grub.cfg That’s the whole bootstrap. The signed grubx64.efi on the ESP has no idea where your real configuration lives, so this stub finds the /boot filesystem by UUID, sets $prefix to point at it, and chains to the grub.cfg that update-grub generates. Three lines, and I’d never read them before the day my NAS stopped booting. ...

My NAS runs on a Supermicro X10SDV-TLN4F — Xeon D-1541, 128 GB of ECC, a 22 TB ZFS pool, and a BIOS dated 2017. It had been up since March. I did some network configuration work one morning, rebooted, and it didn’t come back. What it showed instead was the Supermicro splash screen with A2 in the bottom right corner, and a keyboard that did nothing. DEL wouldn’t get me into Setup, F11 wouldn’t open the boot menu, F12 wouldn’t PXE. The machine sat there indefinitely. ...

I rebuilt a NAS onto a mirrored encrypted root: two SSDs, mdadm RAID1, LUKS2 on top, LVM inside. That layout is well-trodden — it’s what the Debian and Ubuntu Server installers produce when you ask for software RAID plus encryption, and the reasoning for putting RAID below LUKS is sound. You encrypt once instead of twice, resync traffic is ciphertext, and rebuilds work on raw blocks below the crypto layer. ...