[Bug 1767889] [NEW] 18.04 installer does not allow to choose legacy install when launched from UEFI live session

[mohican]

Public bug reported:

Due to a limitation of my BIOS I can only boot from a USB-bootable key
in EFI mode.

However on my machine the OS must be installed in legacy (BIOS) mode.

When I installed Ubuntu 16.04 from this EFI-booted USB-key, I had
warning message from the installer, before choosing the partitioning
option that said approximately :

"The microcode has started the installer in UEFI mode but there seem to
be other OS installed using the BIOS compatibility mode [...] you can
force installation in UEFI mode but you may not be able to start the
other OS. If you want to be able to start them you should NOT force
installation in UEFI mode."

Then hitting the "Back" button yielded to the partitioning option screen
and the installation went on in legacy mode.

But when I tried today to install Ubuntu 18.04 from the downloaded and verified desktop 64 bits ISO, in the same environment, using the same method, there was NO WARNING and therefore in the end of the installation process I had a failure message stating that :
"grub-efi-amd64-signed failed to install to /target/"

The installer failed to detect and to warn that there was other OS
already installed in BIOS legacy mode.

** Affects: grub-installer (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: bios uefi

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https://bugs.launchpad.net/bugs/1767889

Title:
  18.04 installer does not allow to choose legacy install when launched
  from UEFI live session

Status in grub-installer package in Ubuntu:
  New

Bug description:
  Due to a limitation of my BIOS I can only boot from a USB-bootable key
  in EFI mode.

  However on my machine the OS must be installed in legacy (BIOS) mode.

  When I installed Ubuntu 16.04 from this EFI-booted USB-key, I had
  warning message from the installer, before choosing the partitioning
  option that said approximately :

  "The microcode has started the installer in UEFI mode but there seem
  to be other OS installed using the BIOS compatibility mode [...] you
  can force installation in UEFI mode but you may not be able to start
  the other OS. If you want to be able to start them you should NOT
  force installation in UEFI mode."

  Then hitting the "Back" button yielded to the partitioning option
  screen and the installation went on in legacy mode.

  But when I tried today to install Ubuntu 18.04 from the downloaded and verified desktop 64 bits ISO, in the same environment, using the same method, there was NO WARNING and therefore in the end of the installation process I had a failure message stating that :
  "grub-efi-amd64-signed failed to install to /target/"

  The installer failed to detect and to warn that there was other OS
  already installed in BIOS legacy mode.

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