executing grub-install failed

Performing a new install on a SSD device (sdc).

Installing xubuntu itself to a new ext4 partition on the drive seems to work fine, but at the end of the installer it crashes with the error:

"Executing 'grub-install /dev/sdc' failed. This is a fatal error"

The drive also has a new 512MB EFI partition created through the installer.

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
CasperMD5CheckMismatches: ./boot/grub/efi.img
CasperMD5CheckResult: fail
CasperVersion: 1.445
Date: Mon Apr 27 15:32:48 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/xubuntu.seed quiet splash ---
LiveMediaBuild: Xubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)