On Sun, 10 Jun 2018 15:08:31 +0100, Rowland wrote in message
<20180610150831.646273f6@???>:

> https://unix.stackexchange.com/questions/252936/grub2-boot-to-a-second-another-hard-disk
>
> Or failing that, google for the answer ;-)

..https://www.dedoimedo.com/computers/grub-2.html has e.g.
8. GRUB 2 recovery
A. Recover from failed boots

--
..med vennlig hilsen = with Kind Regards from Arnt Karlsen
...with a number of polar bear hunters in his ancestry...
Scenarios always come in sets of three:
best case, worst case, and just in case.