Installation from 17.10 to 20.04 has failed 3 times now and I cannot access any OS

[Ian Smith]

I had another OS installed, and have had 17.10 installed for a number of years without issues - which was an upgrade from maybe 16.04.
I have now tried 3 times to install 20.04 as a clean install over 17.10, and both those attempts resulted in the system rebooting into grub rescue (but no clues as to how to progress from there).
I then tried a clean install over my disk, and having selected the entire disk, the installer crashed.
The USB stick I am using worked fine when used the other day with a Thinkpad, which is why I was installing my work laptop today.
This is the only installation issue I have had, and I have installed Ubuntu on at least 7 PCs and been running it since c. 2008 - or earlier.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15
ProcVersionSignature: Ubuntu 5.4.0-26.30-generic 5.4.30
Uname: Linux 5.4.0-26-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.445
CurrentDesktop: ubuntu:GNOME
Date: Sat Jul 4 10:41:22 2020
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed initrd=/casper/initrd quiet splash --- maybe-ubiquity
LiveMediaBuild: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
ProcEnviron:
 LANGUAGE=en_GB.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)