How to make an EFI Win10 boot instruction in EFI partition?

Hi all.

There are hundreds of posts in these forums and on the internet about EFI and Windows 10 booting, but I can't seem to find a single one that describes how to boot Windows from an EFI partition.

Short Background
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I have a 512GB SSD with Windows 10 Pro installed, booting under MBR/Legacy (upgraded from Windows 8, and upgraded from Windows 7 prior to that, and I don't remember if that was also upgraded for Windows XP). But the point is, I didn't learn about GPT & UEFI until yesterday.

I've installed a new video card, and apparently I could not see anything on the screen upon boot - unless I change the BIOS system setup to "UEFI + CSM". With Legacy mode I couldn't get past the blue system setup screen.

However, my SSD was partitioned in MBR. I have no operating system and I have no other computer I could use - all the computers at work that I can borrow are all running Suse Linux or Ubuntu.

I have a Ubuntu 16.04 Live usb drive that I could use. So I booted from the Ubuntu Live usb drive in UEFI mode and I can get past the blue system setup screen. In fact typing this from the Ubuntu Live system.

Reading online, I learned that I could convert my MBR partition to GPT using the linux tool gdisk. So I did exactly that. I now have 0.2GB of EFI partition in FAT32, about 500GB of MSFT partition, and about 1GB of recovery partition.

But I can't boot my Windows 10.

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What and how do I install information into that EFI partition?

I understand I need to create a bootmgr.efi file or something and copy it into the EFI partition under /boot/efi, but I have no idea how to do this. And how do I tell the bootmgr.efi file to look for my Windows installation in the primary drive in 2nd partition? (In Linux the Windows partition appears as /dev/sda2)

All the instructions I've read over the past 36 hours point to installing Windows from fresh - which I can't do because there are so many things and licenses in the drive that I cannot get back.

Please help.

Have you got an install dvd for windows 10 to do a repair? Restore/Repair Deleted EFI Boot Partition in Windows 10/8/7 – EaseUS

Make a bootalble USB flash drive from Kyhi's Recovery Tools and it will have all you need.

Windows 10 Recovery Tools - Bootable Rescue Disk - Windows 10 Forums

Hi,

Thanks for your reply. I was going through that website yesterday, but it downloads as an exe file and I think it requires to be currently running a Windows 10 operating system to run their file.

I have a Windows 10 install ISO (3.6 GB) but when I run it, I have option either to "Install Windows 10" or advanced settings. Under Advanced settings I could restore from an image, restore point or try to fix boot problems. I've tried them all.

NavyLCDR said:

Make a bootalble USB flash drive from Kyhi's Recovery Tools and it will have all you need.

Windows 10 Recovery Tools - Bootable Rescue Disk - Windows 10 Forums

Thank you, I'm downloading it now, at 4%

Samuria said:

Have you got an install dvd for windows 10 to do a repair? Restore/Repair Deleted EFI Boot Partition in Windows 10/8/7 – EaseUS

Hi Samuria

I must thank you for that link. Although I've already been to that link (it didn't work, it required a running Windows installation, and the last (manual) part (BCDBOOT) gave me an error that it couldn't copy the boot files over.
I revisited that step and searched for other clues around the internet and I have found a solution that worked for me.
Here is what I did, (modified from BSOD on Boot 0xc0000034 Solved - Windows 10 Forums)

1. Boot the computer using a Windows 10 installer usb. You will have to download a windows 10 .iso file and make a usb bootable drive from the .iso file. I found the .iso file on my portable drive, it's an old version (3.5GB) but it works - I didn't need to install it, I just needed it to boot to command prompt.
2. When your installer takes you to the Windows Setup page where you select language and regional settings, press Shift+F10 to drop to command prompt.
3. At the command prompt X:\Sources> type diskpart
DISKPART> list disk
#identify the disk you want to boot from from its capacity (GB), let's say it is disk 0
DISKPART> select disk 0
DISKPART> list partition
#find the partition you will use as the EFI partition. It should be named System with format FAT32 and >100MB & <250MB. Assume this is Partition 1.
#if it doesn't exist, you'll need to create one. Use Ubuntu LiveCD app called Gparted (built in with recent Ubuntu Live CDs)
DISKPART> select partition 1
#because this partition is broken (that's why you're reading this), you'll want to remake it. BE CAREFUL - MAKE SURE IT IS THE <200MB PARTITION THAT IS SELECTED
DISKPART> delete partition
#Your System/EFI partition should be empty space now.
DISKPART> create partition efi
#This creates a new proper System partition
DISKPART> format quick fs=fat32
DISKPART> list partition
DISKPART> list volume
#Write down the drive letter where Windows is installed. It should have a letter, for example Ltr C, means it is on C:\
#now we select the volume with the recently created EFI system. Let's say it is Volume 1.
DISKPART> select volume 1
#now we assign the EFI volume a drive letter
DISKPART> assign letter=b:
DISKPART> exit
4. Now we transfer Windows boot information into this newly created drive letter.
X:> b:
b:> mkdir EFI
b:\> cd EFI
b:\EFI> mkdir Microsoft
b:\EFI> cd Microsoft
b:\EFI\Microsoft> mkdir Boot
b:\EFI\Microsoft> cd Boot
b:\EFI\Microsoft> bootrec /fixboot
#this may fail, I don't recall now, but the next line is the magic
b:\EFI\Microsoft> bcdboot c:\Windows /l en-GB /s b: /f ALL
#if this successfully completes, the next time you type "dir" you should see your Microsoft EFI files listed in this directory. Change en-GB to en-US or ja-JP accordingly.
5. Reboot your computer, selecting EFI boot mode instead of MBR/Legacy. For me, I had to choose UEFI+CSM rather than Legacy.
You should be able to boot into your Windows 10 installation now, meaning your EFI have been recovered.
These notes are for me to revisit just in case one day - but if it helps others then I'll be glad.
Cheers!

You don't have to create all those folders/directories manually, the bcdboot command will create the required folders.