I have found how to get it to install, I removed the other (SATA SSD) disk drive. It now boots successfully, phew!
I've no idea why that second drive breaks things. I installed it when I was still running xubuntu 24.04 and that OS could see the drive OK. I actually copied the whole of my old (xubuntu) installation across onto that drive. I will try putting it back later to see if it breaks the Debian 12 installtion but for the moment I'm just relieved I've got it working at last! Thanks for all the help everyone. I'll try and investigate why the drive broke things but that can wait until tomorrow, I'm worn out watching installations! :-) -- Chris Green
