Win10 + Xubuntu 20.04: Dualboot UEFI issues

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Win10 + Xubuntu 20.04: Dualboot UEFI issues

Post by peter_b »

Fresh install of Win10, then fresh install of Xubuntu 20.04.3: Both installed, but no boot manager/option shows up to select Ubuntu - Windows just boots as if noone else was there. :?

I've searched the web and it seems to be a common problem, but possibly due to different reasons.

Therefore, everything I've tried didn't work:
  • Disabled non-UEFI (legacy) boot options in BIOS. Then reinstall Xubuntu. Just to make sure that the first installation wasn't a legacy-MBR one by accident. Same result.
  • boot-repair: bailed out with a strange error message, saying Windows was not shutdown but merely hybernating (*).
  • bcdedit: copy Windows entry and modify it to point to \EFI\ubuntu\shimx64.efi (or grubx64.efi). Both didn't work (Error code 0xc000007b).
  • etc etc... aarrrgh!
The strange (but good) thing is that when selecting the "ubuntu" EFI boot entry from the BIOS directly: Voila! it boots Linux as expected.
So I knew the installation basically works.

My BIOS (MSI Mortar Max B450M Mainboard) had "BIOS CSM/UEFI Mode" selected, instead of just "UEFI".
CSM stands for "Compatibility Support Module". Wikipedia says:
The Compatibility Support Module (CSM) is a component of the UEFI firmware that provides legacy BIOS compatibility by emulating a BIOS environment, allowing legacy operating systems and some option ROMs that do not support UEFI to still be used.
Long story short: 3 things to do.
  1. Disable "fast startup" in Windows10.
  2. Set BIOS mode to UEFI (disable CSM).[/b]
  3. Set the default UEFI boot order in the BIOS to default to "ubuntu".
1. (*) The first one is enabled by default, making Win10 boot up faster by never really shutting down (what an ugly hack is that? :shock:). That was also the reason why boot-repair correctly complained that Windows was merely hybernating.

2. Makes thing do "clean UEFI". No legacy fallback hacks coming in the way.

3. This loads GRUB by default, which should have "Windows 10" as option in its menu :)
Windows' boot loader seems to have issues loading non-Microsoft kernels. Surprise?
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