Grub installation failed during Ubuntu installation, readonly filesystem

[Michael Duckwitz]

Tried to install Ubuntu alongside Windows 10. It failed when it tried to install Grub. The error said the target location was a read-only filesystem.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: ubiquity 21.04.19
ProcVersionSignature: Ubuntu 5.11.0-16.17-generic 5.11.12
Uname: Linux 5.11.0-16-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu65
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.461
Date: Wed Aug 18 22:36:10 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 21.04 "Hirsute Hippo" - Release amd64 (20210420)
RebootRequiredPkgs:
 linux-image-5.11.0-16-generic
 linux-base
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)