OK, maybe I was a bit harsh with the verdict on mounting by UUID in my last post. Having played further with trying to get the right disk to boot by default, I realised that there is something rather odd about my ASUS motherboard.
I seems to want to boot SATA drives in REVERSE order so that SATA disk 3 is booting before SATA disk 1 no matter what order I tell it boot in (via the BIOS).
So I’m back to using UUID’s in both GRUB (where possible) and fstab. And all seems to be working again.
None of this is beginer friendly though! I don’t know even that many professionals who wouldn’t give up on this stuff. It’s only fine if you only have a single hard drive.