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So I have encountered the problem above mentioned. It might not be specific to Ubuntu, but it is Linux in general. The specifics are that I have created a separate 20gb partition for its installation and installed in legacy mode.

After the installation, I'm able to use Fedora well but only in Legacy mode (obviously).

When I change the BIOS, which I am able to access somehow, to UEFI, it is mentioning that no disk was found, and hence I cannot boot windows.

In Fedora when I check the partition, every bit of data is intact in C, D, and E drives regarding windows files, i.e. same before and after installation. I even checked the partitions and found that Microsoft and Windows files are there. So I'm assuming I have not replaced or tampered with Windows 10 OS.

There is a separate partition with Linux (20gb one). It consists only of Linux installed files.

Can anyone please help with restoring the Windows 10 OS back??

PS: I don't necessarily need Fedora to still be there, if uninstalling is required I'll do it, just need Windows back

migrated from askubuntu.com Jun 1 '17 at 9:03

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    A legacy install of anything should have no effect on a Windows UEFI install. Just put your machine in UEFI mode and boot Windows. Select Windows in the EFI menu (fct key at power up) is always available too if necessary. – ubfan1 May 30 '17 at 16:14
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As you've probably realized by now, installing Fedora in BIOS/CSM/legacy mode to a computer that already had an EFI/UEFI-mode Windows installation was a mistake. My page on the CSM describes some of the problems that this type of configuration can create. Thus, one of your goals should be to either re-install Fedora in EFI mode or convert the existing installation to boot in EFI mode. Fortunately, converting Fedora to boot in EFI mode is relatively straightforward; you need only install an EFI-mode boot loader. Ubuntu users often turn to the Boot Repair tool, but I don't know how well it would cope with Fedora. Alternatively, you could install an EFI-mode GRUB, or some other EFI-mode boot loader, manually. See my page on EFI boot loaders for Linux for a (somewhat old) rundown of what's available. My own rEFInd boot manager is likely to be fairly easy to install -- you can use the USB flash drive or CD-R image to do a one-time boot and then install the RPM in Fedora. One caveat about rEFInd is that you need to jump through some extra hoops to get it working with Secure Boot enabled.

Your bigger problem, though, seems to be your inability to boot Windows. You wrote:

When I change the BIOS, which I am able to access somehow, to UEFI, it is mentioning that no disk was found, and hence I cannot boot windows.

Technically, you have UEFI firmware, not BIOS. Many people, and even manufacturers, apply the term "BIOS" to EFI/UEFI firmware, but in my experience this just creates confusion; EFI is very different from BIOS in how it boots the computer, so calling an EFI a BIOS leads people to drag in BIOS assumptions that lead to trouble when applied to EFIs.

That said, your claim that "it is mentioning that no disk was found" is vague. It would be helpful to see a screen shot (a digital photo is fine) of this error message so that we can see the exact wording, fonts, etc., all of which can provide clues about what's producing this message -- the firmware, a boot loader (and which one), a Windows component, etc.

My suspicion is that you haven't completely reversed the changes you made to the firmware. Two specific thoughts on this occur to me:

  • In some cases, changing one firmware option automatically makes changes to more options, so reversing that one change may require making additional changes. Unfortunately, I can't really be sure what additional options you'd have to change.
  • Windows is sensitive to the firmware's disk access method. Typical options include "AHCI," "IDE," and "RAID." If you changed this firmware option, be sure to change it back. This would not normally be changed automatically when switching from native EFI-mode booting to BIOS/CSM/legacy-boot mode, but it's conceivable that your computer is an exception to this rule.

It should also be noted that, depending on the nature of the problem with the Windows boot, it's conceivable that installing an EFI boot loader for Linux might help, since this boot loader is also likely to serve as a boot manager, which should pass control to the Windows boot loader when you tell it to. (This would help if the problem is that the firmware has "forgotten" the Windows boot entry, which might happen if you unplugged the disk or if the firmware is stupid enough to wipe the EFI-mode boot entries when you reconfigured it to boot in BIOS mode.)

EDIT: For background information to help you understand my answer, please read:

  • Sir, when you mention the installation of an EFI-mode boot loader, I'm not quite able to understand what you mean. I only have the ability to access Fedora through a legacy boot and an EFI boot loader works in UEFI if I'm correct. Unfortunately, I can't use my flash drive to boot Fedora in UEFI as system is blocking me out. Also I'm unable to disable secure boot without Windows, which is my initial problem (Secure boot option is grey in colour). I tried using rEFInd before but I'm not sure it works in Legacy. On the other hand, I'll upload the screenshot in some time (no disk found). – Karthik C Jun 2 '17 at 6:03
  • Please see my edit, above. – Rod Smith Jun 3 '17 at 1:34
  • Sorry for the late reply sir. The "no disk" problem was the case that my hard drive was recognised but boot caused problems. I think the EFI installation of Linux would have definitely helped but I found out the BIOS menu security disable after I clean installed Windows 10 again. All the options were the exact same in BIOS with HDD set to AHCI as per usual. As you mentioned Linux might have provided a bootloader for windows. I will be trying to reinstall Linux in a few days again using UEFI. Thanks a lot for the information you have given on your page and your EDIT. – Karthik C Jun 12 '17 at 17:16
  • It will surely help with reinstalling Linux the right way this time without overwriting windows. – Karthik C Jun 12 '17 at 17:20

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