[Bug 1786992] Re: Existing bios mode Windows install left unbootable after installing in EFI mode

[Phillip Susi]

To clarify, if Ubuntu is booted in EFI mode, os-prober will currently
assume that grub will continue to boot in EFI mode and so refuse to add
a bios Windows chainloader stanza to the menu since it will not work.
It will only add a windows entry if it finds the windows efi loader in
the ESP.  It may be able to come up with sufficient logic to have a bios
chainloader entry that is only displayed by grub-pc and hidden in grub-
efi, and a corresponding EFI mode that is hidden vice versa.

That would get us out of this mess at least with Windows, though bare
metal FreeDOS installs are still going to be left out in the cold.

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https://bugs.launchpad.net/bugs/1786992

Title:
  Existing bios mode Windows install left unbootable after installing in
  EFI mode

Status in os-prober package in Ubuntu:
  Triaged
Status in partman-auto package in Ubuntu:
  Triaged
Status in partman-efi package in Ubuntu:
  Triaged
Status in ubiquity package in Ubuntu:
  Triaged

Bug description:
  Bugs #1668148 and #1766945 changed the installation process to install
  grub-efi instead of grub-pc when the installer is booted in EFI mode
  but the existing OS on the disk is in bios mode.  Currently, this
  renders the existing Windows or FreeDOS install unbootable.  Windows
  installs at least may be able to be salvaged by patching os-prober to
  generate a grub.cfg that still has the windows bios mode chainloader
  command when interpreted by grub-pc ( if the user tells their firmware
  to boot in legacy mode ), or to chainload the windows EFI boot loader
  when booting grub-efi.  Normally os-prober looks for the windows
  bootmgr in the ESP, but in the case of a bios mode install, a copy can
  also be found in the windows\boot directory and at least in theory
  should be loadable to boot Windows even though it was originally
  installed in bios mode.

  As things currently stand, this will be a major breakage for users in
  18.10 and 18.04.1, so unless the changes to os-prober can be made in
  time, the other two changes should be reverted and simply stick with
  grub-pc in the case where the existing OS is in bios mode.

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