Installation of Ubuntu Unity 20.10 always fails if there is no EFI partition

[Liam Proven]

Even on BIOS systems with no UEFI

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
CasperMD5CheckMismatches: ./pool/main/b/binutils/binutils-common_2.35.1-1ubuntu1_amd64.deb
CasperMD5CheckResult: skip
CasperVersion: 1.455
Date: Tue Feb 9 15:38:57 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz boot=casper file=/cdrom/preseed/ubuntu.seed maybe-ubiquity ignore_uuid quiet splash ---
LiveMediaBuild: Ubuntu Unity 20.10
RebootRequiredPkgs:
 linux-image-5.8.0-25-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Liam Proven (lproven)]

Confirmed and reproduced in Xubuntu 20.10 as well. This issue is _not_ confined to Ubuntu Unity and is also present in an official remix.

Steps taken to try to resolve it:
* updated system BIOS (machine is a Lenovo Thinkpad W500; was on 3.18, now on 3.23, latest) -> no change
• tried 2 different flavours of 20.10 -> no change
• placed a bootable DOS primary partition (C:), set active, tested OK; no change.
• installed Windows 7 Enterprise SP1. No UEFI detected, no EFI partition created, no separate system partition needed or used.

Ubuntu Unity 20.04 went on this machine without a glitch. openSUSE Leap 15.2 was also fine.

The machine does not have UEFI, as far as I can tell. There is no option to boot in legacy or UEFI mode.

The only way I have discovered to install 20.10 was to tell it my DOS partition was the EFI System Partition. This worked fine, installed GRUB into /dev/sda and now it boots.

However this has rendered my DOS partition unbootable, and now it is mounted at /boot/efi which is not what I want.