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

[Chris Green]

The continuing saga of trying to get xubuntu 19.10 working on a NVME
SSD.

Just to be on the safe side I unplugged all my 'important' disk drives
(i.e. the xubuntu 19.04 installation and my /home), this leaves just
the new NVME SSD and my 'spare' 500Gb SATA spinning drive.

I run the xubuntu 19.10 installation selecting "something else" at the
disk partitioning stage.  I have partitioned as follows:-

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

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

... and I say mount /boot on /dev/sda1, / on /dev/nvme0n1p2.

The installation proceeds and does all the usual things, copies files,
installs stuff, etc.  The 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!

In addition it *has* installed something on /dev/sda which prevents my
xubuntu 19.04 from booting (and, yes, I did plug all the drives back
in).  Fortunately unplugging /dev/sda has let 19.04 boot now, so I'm
back working but not much nearer getting 19.10 installed.

-- 
Chris Green