Ubuntu 20.04.1 installer crashed

[Glenn Stewart]

Could not create GRUB- this is a fatal error was the gist of the message displayed.

I was installing Ubuntu from a flash drive.
Installer did not recognise that Win 10 was already installed.
Installed Ubuntu on a secondary disk that had unallocated space.
I want to be able to dual boot between Ubuntu and Win 10, as I migrate to Ubuntu.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15.2
ProcVersionSignature: Ubuntu 5.4.0-42.46-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.4
Architecture: amd64
CasperMD5CheckResult: skip
CasperVersion: 1.445.1
Date: Sun Dec 27 12:00:22 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)