xubuntu 19.10 fails to install with /boot on different drive - grub-install error

[Chris Green]

I am trying to install xubuntu 19.10 on an NVME SSD which isn't
visible to the BIOS.

So I need to put /boot on a drive which *is* visible to the BIOS, I
have drives as follows:-

    /dev/nvme0n1
        /dev/nvme0n1p1     2048   31250431   31248384 14.9G Linux swap
        /dev/nvme0n1p2 31250432 2000408575 1969158144 939G Linux filesystem

    /dev/sda1
        /dev/sda1                                      4xxG Linux filesystem


I'm installing from a USB stick, the installation starts OK, when I
get to the disk allocation bit I select 'something else' and tell it
to mount / on /dev/nvme0n1p2 and /boot on /dev/sda1 (which is a SATA
drive visible to the BIOS).

The installation proceeds and does all the usual things, copies files,
installs stuff, etc.  Then over half-way through it pops up an error
box:-

    Executing 'grub-install /dev/nvme0n1' failed
    This is a fatal error

The install is then totally broken as none of the options to get past
this point actually work when you click the buttons!

The options it suggests are:-
    Try grub-install to a different drive, presumably this is what I
    need to do, but selecting /dev/sda and clicking the button does
    nothing.

    Continue without installing boot - clicking this does nothing

    Abort the installation - clicking this does nothing

The only way out is to power down!

Help!  How can I get any further?
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Chris Green