Re: problems dual booting Ubuntu 16.04 and Windows 10

[Pascal Hambourg]

Le 11/05/2018 à 18:05, David Collier a écrit :
> Disk /dev/sdc: 465.8 GiB, 500107862016 bytes, 976773168 sectors
> Units: sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
> Disklabel type: gpt
> Disk identifier: AF6118F0-6BF8-460C-9622-F5BDA7EE3204
>
> Device       Start       End   Sectors   Size Type
> /dev/sdc1     2048   1023999   1021952   499M Windows recovery environment
> /dev/sdc2  1024000   1226751    202752    99M Microsoft basic data
> /dev/sdc3  1226752   1259519     32768    16M Microsoft reserved
> /dev/sdc4  1259520 976773119 975513600 465.2G Microsoft basic data
(...)
> /dev/sdc2: SEC_TYPE="msdos" UUID="A879-04E5" TYPE="vfat" PARTLABEL="EFI
> system partition" PARTUUID="2987d536-85c4-4a68-ad8a-7c880054bda9"

At first I was surprised not to see an EFI system partition on sdc, but 
here it is, sdc2. It just has the wrong partition type identifier in the 
partition table, which should be "EFI system partition" instead of 
"Microsoft basic data". You should correct this with your favourite 
GPT-capable partition editor (gparted, parted, fdisk, gdisk...)

> > This means that the system did not boot in EFI mode. It explains why
> > os-prober cannot detect Windows EFI boot loader.
> > Check what's in /boot and /boot/grub : I guess there is no /boot/efi
> > directory nor /boot/grub/x86_64-efi, but there is a /boot/grub/i386-pc
> > directory.
>
> yes, these guesses are correct.  So, is there a way to make grub include
> windows 10 option in this case? How come the BIOS/UEFI, (whatever my PC's
> firmware is that is :) can boot both, but grub can not?

Windows was installed for EFI boot, and I do not know any way around 
this except by reinstalling it for BIOS/legacy booting. So you have to 
install the GRUB boot loader for EFI booting (GRUB EFI). The GRUB 
distribution is split in multiple packages for all supported targets and 
usually you need to install the package grub-efi-amd64, but since you 
built GRUB from source, I'll assume the needed files are already present.

First, you must mount the EFI partition sdc2 on /boot/efi (create the 
directory if it does not exist, and make the mount persistent in fstab - 
as usual, use the UUID, not the unreliable device name).

Then ideally you must find a way to boot in EFI mode and start a shell 
in Ubuntu. For example boot from an EFI-capable live system or whatever, 
chroot into Ubuntu's root and mount everything required (/dev, /proc, 
/sys, /boot/efi...). efibootmgr should not complain about EFI variables 
not being supported.

From there, install GRUB EFI :

grub-install --target=x86_64-efi --bootloader-id=Ubuntu

This will install GRUB EFI as /boot/efi/EFI/Ubuntu/grubx64.efi and will 
try to register a boot entry for it in the UEFI firmware NVRAM. If it 
succeeds, efibootmgr should show a new "Ubuntu" entry, and it should 
also be present in the UEFI boot menu. While in EFI mode, os-prober 
should detect Windows Boot Manager and you can run update-grub to 
include Windows in GRUB's menu.

If you cannot run Ubuntu in EFI mode, here are a few workarounds :

a) Some UEFI firmware allow to create a custom boot using a given EFI 
boot file. Select \EFI\Ubuntu\grubx64.efi on the drive corresponding to sdc.

b) Install GRUB EFI as the fallback boot loader with

grub-install --target=x86_64-efi --removable

and, in the UEFI boot menu, choose to boot from the drive corresponding 
to sdc in EFI mode (may be referred to as "EFI shell")

c) Rename the Windows boot loader /boot/efi/EFI/Microsoft/bootmgfw.efi 
and replace it with a copy of grubx64.efi. Yes, that's ugly but is 
sometimes the only way to boot in EFI mode the first time. After 
reinstalling GRUB EFI without error, restore the original bootmgfw.efi 
so that you can boot Windows again.