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Some days ago, I installed Windows 10 to a new HDD. Afterwards, I found two boot options in firmware: (1) HDD, (2) UEFI - HDD. I thought the former is boot in legacy mode, the latter is boot in UEFI mode. I tried both and could boot to Windows.

Since then, I installed another OS & moved the files inside the EFI system partition (ESP) & reverted back... I forgot what I did exactly. Now, the firmware shows (1) HDD, (2) Windows - HDD. Option 2 can boot to Windows. But when I select Option 1, the system halts in blank screen. I think that means Windows can boot in UEFI but not legacy mode now.

Is it possible to make Windows bootable in both UEFI and legacy mode at this moment, without losing data?

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Chances are you were booting in just one mode all along. One of the problems with the built-in EFI boot managers is that they don't always do what you think. When you select Option A in the boot manager, it might try that option, but then if that option fails, it might start going through its regular boot order, so you might end up booting via Option B or Option C even though you selected Option A. (I've been frustrated by this tendency myself on more than one occasion. I keep screaming at the computer to do it what I tell it to do, but it never listens. ;-) ) Thus, your attempt to boot in two modes might have ended up booting in just one mode.

AFAIK, it's not possible to install Windows in a way that will boot a single installation in either BIOS/CSM/legacy mode or in EFI/UEFI mode simply by selecting a different boot option. If it is possible, and if that really is what you've accomplished, chances are nobody else (except possibly some Microsoft developers) could tell you how to do it.

  • After years of yelling at computers to no avail, I have found a method that works! If you open a compiler, and yell text at it, sometimes it will even respond with a syntax error! – Blaine Jul 7 '16 at 3:22
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It is actually quite easy to make a UEFI Windows 10 installatzion bootable in BIOS mode as well:

First you need to create a hybrid MBR from the GPT (e.g. using gdisk), including the Windows partition (type 7) and making it bootable.

Then boot a Windows install CD/USB stick in BIOS mode and use startup repair.

This will create a bootblock in the windows parition that allows it to boot in BIOS mode, using e.g. grub's chainloader (hdx,gptx)+1.

To get it to boot in BIOS mode without any bootloader, you need to install suitable MBR boot code (e.g. uswing install-mbr), that will chainload into the active partition.

The windows installation will still be bootable in UEFI mode, however, it is likely that (unlikely) updates to windows boot components will only affect the currently-booted mode.

  • A more detailed explanation/tutorial would be extremely helpful – Nikita 웃 Jun 26 '18 at 20:10

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