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I've first installed Windows 10 and afterwards the latest Xubuntu version. If I boot now it directly boots Xubuntu. Grub won't be loaded. I also changed the boot style in grub from hidden to menu without success. Then I tried to press the esc key during startup to force the grub boot menu without success either. So it looks like grub isn't loaded at all.

When I change the boot order (in UEFI) to the hard disk where windows 10 is installed, it boots without any problems. I tried out the tool boot-repair. When I chose apply it said that I need to enable legacy mode for CRM. I checked in the UEFI screen and it's already activated. The error I get from boot-repair is LegacyWindows detected.

When installing Xubuntu I created an efi partition as this was required. I see this partition in my fstab. How do I get dual-boot working without having to go into Uefi-Bios to select the disk to boot?

Daniel
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    If you installed Windows in BIOS/Legacy/CSM mode you cannot dual boot from grub, only from UEFI boot menu. UEFI & BIOS are not compatible and once you start to boot in one mode or other, you cannot switch. It sounds like you installed Windows in old BIOS mode. Microsoft has required vendors to install Windows in UEFI/gpt mode since 2012. User can select BIOS/MBR but should not. How you boot install media, is then how it installs. Best to reinstall Windows in UEFI mode to gpt partitioned drive. – oldfred Sep 26 '21 at 20:20
  • Does this answer your question? Convert from EFI to BIOS boot mode and Unable to boot into Windows after installing Ubuntu, how to fix? – karel Sep 26 '21 at 23:32
  • It would work but I prefer the other solution to upgrade the Windows installation to UEFI. This worked for me. It's a bit weird as it seems there are two uefi boot managers now. So I go from Ubuntu to Windows which is a bit slow. – Daniel Sep 28 '21 at 19:34

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You can't have dual boot between lagacy and uefi systems.

The easiest way to fix that would be to convert Windows boot from Legacy to UEFI mode. There's a step-by-step I found on that how to do that: https://www.maketecheasier.com/convert-legacy-bios-uefi-windows10/

If you want to reinstall windows from fresh iso, you will need to change partition table type using other tools. There's a very good one I've actually used several times before called gdisk: https://www.explorelinux.com/convert-disk-mbr-to-gpt-on-linux/

Before you start - back up your data. If something goes wrong, you'll probably loose everything on that drive.

After you're done run Ubuntua and run grub os-prober to find Windows (in case it was not found automatically). Alternativelly you can install rEFInd boot manager - I find it to be much better than grub for dual-boot efi systems.

Piotr X
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