Failed Debian 12 install, need help with boot loader re-install

[Chris Green <cl@isbd.net>]

I just tried to install Debain 12 onto my Fujitsu Esprimo Q957 system
(was running xubuntu previously).  I have installed Debian 12 using
the same USB stick on two other systems so the installation media are
OK.

The whole installation ran without any problems but it simply fails to
boot, I just get a blank black screen with a prompt at the top left
cormer.

I'm attempting to re-install the boot loader using the graphical
rescue from the USB stick but it's not at all clear which partition I
should be installing it on.

The system has two SSD drives - /dev/nvme0 and /dev/sda.  I'm
installing Debian on /dev/nvme0.

When I go to re-install the boot loader I'm offered /dev/nvme0n1p1,
/dev/nvme0n1p2 or /dev/nvme0n1p3 (plus /dev/sda of course but I don't
want it there) but there's no indication which one of these I should
put the boot loader on.

On one of the systems where I have installed Debian 12 without
problems it has:-

    Filesystem                Type 1M-blocks   Used  Avail Use% Mounted on
    /dev/mapper/t470--vg-root ext4    936644 197158 691835  23% /
    /dev/nvme0n1p2            ext2       456    121    311  29% /boot
    /dev/nvme0n1p1            vfat       511      6    506   2% /boot/efi

Presumably the new/failed install will have a similar configuration

The xubuntu installation worked fine from /dev/nvme0 so I don't think
there's can be anything fundamentally wrong.  Are there any BIOS
settings I should check?

Thanks for any/all help.

-- 
Chris Green