Grub

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. ...