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I am using Ubuntu 20.04 LTS along with windows. Recently upgraded from Windows 10 to 11. Now the GRUB boot menu is lost and the laptop's booting by default to Windows 11
How to recover grub?
I have an SSD for Ubuntu and a HDD for windows i.e., they are both on different hard drives.
That Windows and Ubuntu are in different drives is irrelevant.
The Windows product upgrade simply changed the boot order to itself.
All you need to do now is open UEFI settings > Boot menu and change it back to "Ubuntu". Then boot Ubuntu and run sudo update-grub to... Update Grub! (probably not necessary - The upgrade Windows 11 should be using the same bootloader - but strongly recommended, just in case).
This is how the boot process works, and consequently how a multi-boot system should be managed, in any modern computer with UEFI, the firmware that replaced the 1981 BIOS more than a decade ago.
PS - If it were a BIOS machine - or Legacy installations in UEFI - not the case here otherwise you wouldn't be able to upgrade to Windows 11 (no longer supports BIOS/Legacy) - then you'd need to reinstall Grub from a live session because it would have been overwritten by the Windows installer.
But with UEFI the boot process is different, many different bootloaders can coexist in the same system, all stored in the ESP (EFI System Partition) and that's why the location of the system partitions of different OSes, in the same or different drives, is now irrelevant.
