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I use Secure Boot, followed by GRUB2, set up to boot Debian 12 and Windows 11 at the choice of a user. Right after the user chooses Windows and hits ⏎, we see a black screen with two white lines on it (the text may be slightly screwed due to retyping from bad screenshots) in the left upper corner:

/EndEntire
file path: /ACPI(a0341d0,0)/PCI(4,1d)/PCI(0,0)/UnknownMessaging(17)/HD(1,800,82000,bc49b1f82df92a47,2,2)/File(\EFI\Microsoft\Boot)/File(bootmgfw.sfi)/EndEntire

The screen contains no further output.

BIOS/UEFI, GRUB when booting Linux, and Windows are not known to be too talkative during boot in modern days in general. So, I guess, the user better pays attention to the above message. Which piece of software issues this message? What does the message tell us?

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This is generated by GRUB.

Since GRUB cannot actually boot Windows by itself, what it does is called "chainloading" where GRUB loads the Windows bootloader and then lets it actually load Windows.

What you are seeing is GRUB going through several different protocols to locate the Windows bootloader called bootmgfw.sfi.

Like with many pieces of text present during Linux startup, this would be helpful information if the chainload attempt fails, as you can then follow every step GRUB took to find the Windows bootloader.

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