Well, as the title implies, I was running update-initramfs -u on my Ubuntu installation after having (i) duplicated the SSD from another existing system, and (ii) having rebuilt a kernel driver (which then required the aforementioned update-initramfs command).

Unfortunately, the command threw back a “W: Couldn't identify type of root file system for fsck hook” error…

So, looking at my /etc/fstab file, I realised I had failed to update it after SSD duplication, which then borked the file because:

• the fstab was referencing devices by /dev/<device+partition> paths instead of by:
  • partlabel (i.e. /dev/disk/by-partlabel),
  • partuuid (i.e. /dev/disk/by-partuuid) or
  • uuid (i.e. /dev/disk/by-uuid)

AND

• I had switched from a SATA SSD to an NVMe SSD, whereby the /dev/sdXN paths were all now /dev/nvmeXnYpN

Any of which the first three path reference types would have worked…

Anyway, a quick fix to the /etc/fstab and all was well again… It was a wonder booting worked at all with the invalid /etc/fstab – UEFI EFS and GRUB saving the day again…