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I had a Windows 10 Installation in Legacy BIOS Mode on my Primary Drive (125GB SSD) and a Secondary Drive (2TB HDD) as a Data Drive (Both Partitioned as MBR).

I created a new Partition on my secondary Drive to install Ubuntu on to. I installed Ubuntu in UEFI Mode (My mainboard seems to support UEFI and Legacy Simultaneously, because I can choose between "UEFI" and "Legacy + UEFI", at least that's what I thought). I created a swap partition, a root partition and an EFI Partition where I told the installer to place the Boot Loader into.

After all of this was done Ubuntu was Booting Fine but there is no Sign of Windows 10 when entering the Grub Menu (By Pressing ESC on startup).

When I try booting directly from the SSD (By changing the boot order) it says something like

Cannot Boot from this Drive.

I Already ran Boot-Repair, but it didn't bring back Windows yet. This is the paste it produced.

I'm running out of ideas and I really don't want to lose my Windows install.
Any help is highly appreciated.

zx485
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Florian Giger
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  • Does you chose in Bios (boot menu) correct sata? – Itra Oct 23 '18 at 19:09
  • and try to update and upgrade grub – Itra Oct 23 '18 at 19:10
  • I tried both of them. And "Boot Repair" does also update/upgrade Grub so i did that too. :/ – Florian Giger Oct 23 '18 at 19:12
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    You cannot easily mix UEFI & BIOS, they are not compatible. You then can only boot directly from UEFI, not dual boot from grub menu. And UEFI needs boot flag on ESP - efi system partition (FAT32), but Windows has to have boot flag on the primary NTFS partition with boot files (your sda1). It seems you may have two Windows installs, sda2 & sdb1? Best at this point to convert Ubuntu to BIOS boot. But you may need to move boot flag to sda1 first, temporarily install Windows boot loader to sda and repair Windows. Then since 2 drives may be better just to have grub in sdb using Boot-Repair. – oldfred Oct 23 '18 at 19:31
  • That sounds promising, how would i go about doing that? Sounds rather complicated. – Florian Giger Oct 23 '18 at 19:34
  • Also it looks like after running Boot-Repair it has now overwritten the Windows Bootloader on sdb's MBR with Grub – Florian Giger Oct 23 '18 at 19:59
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    Possible duplicate of Convert from EFI to BIOS boot mode – karel Dec 05 '19 at 09:00

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So it appears using Boot-Repair and writing Grub into the MBR of the Drive you can turn the UEFI Ubuntu installation into a legacy one.

When Booting up Ubuntu in Legacy Mode now 'os-prober' finds the Windows installation and running 'update-grub' it now adds a menu-entry for windows and I can now Dual boot into Windows and Ubuntu.

The Conclusion and the Lesson learned from this is: Don't Mix UEFI and Legacy Mode OS installs.

Although I don't know if I were to remove the Ubuntu Installation again if my regular Windows Boot Loader would still be working. I don't actually think so.

Florian Giger
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