grub-installer failed; ubuntu hirsute on BIOS box; replace a partition

Ubuntu hirsute (daily) QA-test install on
- dell [optiplex] 755 (c2d-e8300, 8gb, amd/ati radeon rv610/radeon hd2400 pro/xt)

Box already had two installs,
- full disk install
- install alongside

This install was just a selection of one of those partitions, selected as "/" and install

I note

Mar 25 03:27:45 ubuntu grub-installer: Installing for x86_64-efi platform.
Mar 25 03:27:56 ubuntu grub-installer: grub-install: error: cannot copy `/usr/lib/grub/x86_64-efi-signed/grubx64.efi.signed' to `/boot/efi/EFI/ubuntu/grubx64.efi': No space left on device.

but this is a BIOS system, not EFI, likely duplicate of https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1907180

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: ubiquity 21.04.11
ProcVersionSignature: Ubuntu 5.11.0-11.12-generic 5.11.0
Uname: Linux 5.11.0-11-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu60
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.461
CurrentDesktop: ubuntu:GNOME
Date: Thu Mar 25 14:36:51 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 21.04 "Hirsute Hippo" - Alpha amd64 (20210324)
ProcEnviron:
 LANGUAGE=en_AU.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_AU.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.11.0-11-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)