New install fails at 'Grub install'

[Chris Pollock]

Trying to install 20.10 from USB created from ISO downloaded, install fails at 'Grub install to /dev/sda'. This happens with both Kubuntu and Ubuntu. I chose 'Something else' at the 'where to install' prompt ('Manual' for Kubuntu), as I have a 256GB GPT SSD on a Thinkpad X201 (BIOS boot) which has two c.45GB partitions on which I alternate Ubuntu releases and a c.150GB as '/home'.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity 20.10.13
ProcVersionSignature: Ubuntu 5.8.0-25.26-generic 5.8.14
Uname: Linux 5.8.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu50
Architecture: amd64
CasperMD5CheckResult: skip
CasperVersion: 1.455
CurrentDesktop: ubuntu:GNOME
Date: Thu Nov 19 21:10:58 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
ProcEnviron:
 LANGUAGE=en_GB.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.8.0-25-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu Foundations Team Bug Bot (crichton)]

Exception during installation:
Nov 19 21:10:26 ubuntu /plugininstall.py: Traceback (most recent call last):
Nov 19 21:10:26 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 1792, in <module>
Nov 19 21:10:26 ubuntu /plugininstall.py: install.run()
Nov 19 21:10:26 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 62, in wrapper
Nov 19 21:10:26 ubuntu /plugininstall.py: func(self)
Nov 19 21:10:26 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 237, in run
Nov 19 21:10:26 ubuntu /plugininstall.py: self.configure_bootloader()
Nov 19 21:10:26 ubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 936, in configure_bootloader
Nov 19 21:10:26 ubuntu /plugininstall.py: raise install_misc.InstallStepError(
Nov 19 21:10:26 ubuntu /plugininstall.py: ubiquity.install_misc.InstallStepError: GrubInstaller failed with code 1
Nov 19 21:10:26 ubuntu /plugininstall.py: