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From: https://answers.microsoft.com/en-us/windows/forum/windows_10-update/windows-10-bootrec-fixboot-access-is-denied/747c4180-7ff3-4bc2-b6cc-81e572d546df

Windows 10 Installation Media:

    Insert the Media (DVD/USB) in your PC and restart.
    Boot from the media.
    Select Repair Your Computer.

Select Troubleshoot.

    Choose Command Prompt from the menu:

Type in the command:

Diskpart

Type in the command:

List disk            (Note which disk is your Boot drive number mine is 0)

Type in the command:

Sel disk 0

Type in the command:

List vol               (Note which volume is the EFI partition mine is 4)

Type in the command:

Sel vol 4

Type in the command:

assign letter=V:

Type in the command:

Exit

Type in the command:

V:

After you have assigned a drive letter Using Diskpart You can format the EFI partition:

Example: if you assigned a letter V to the partition the command would be:

format V: /FS:FAT32

After the format you need to recreate the EFI directory structure with the command:

MD \EFI\Microsoft\Boot

Then change to the Boot directory with:

cd /d V:\EFI\Microsoft\Boot\

Then run:

bootrec /FixBoot (if -Access Denied- See correction below)

Finally run:

bcdboot c:\Windows /l en-us /s V: /f All

Type in the command:

bcdboot C:\windows /s V: /f UEFI      (This replaces the above crossed out lines and works in Win 10 1709)

Remove This

After the format you need to recreate the EFI directory structure with the command:
MD \EFI\Microsoft\Boot
Then change to the Boot directory with:
cd /d V:\EFI\Microsoft\Boot\
Then run:
bootrec /FixBoot
Finally run:
bcdboot c:\Windows /l en-us /s V: /f All

A D D   T H I S 

Type in the command:

bcdboot C:\windows /s V: /f UEFI      (This replaces the above crossed out lines and works in Win 10 1709)

From <https://answers.microsoft.com/en-us/windows/forum/all/windows-10-bootrec-fixboot-access-is-denied/747c4180-7ff3-4bc2-b6cc-81e572d546df>

I struggled with this error too. I got it working by booting for an USB drive created with Rufus. A Windows DVD or another bootable medium would work too. Maybe the path changes in that case. I verified if the BCD-Template file exists from the command prompt (which in my case didn't) by issuing: dir %WINDIR%\System32\Config\BCD*

Then I copied the BCD-template file from the Rufus disk (X:) by issuing: xcopy X:\windows\System32\Config\BCD-Template %WINDIR%\System32\Config\BCD-Template

From <https://superuser.com/questions/713146/windows-8-bfsvc-error-could-not-open-the-bcd-template-store-status-c000000>

Step 9. Type assign letter=N: and press Enter. N: can be replaced by any drive letter that is not taken by any partition. Remember which drive letter you assign here for it is rather useful in the rest steps. Step 10. Type exit and press Enter to leave diskpart. Step 11. Now type N: (the drive lettered you just assigned) and hit Enter. Step 12. Type format N: /FS:FAT32 and hit Enter. Step 13. Type bcdboot C:\windows /s N: /f UEFI and hit Enter. Now repeat the process of running the bootrec /fixboot command, bootrec access is denied Windows 10 should be solved now. But if fixboot access is denied occurs on an MBR disk, continue with Fix #2.

From <https://www.partitionwizard.com/clone-disk/bootrec-fixboot-access-is-denied.html>

From: https://www.dell.com/support/article/us/en/04/sln300987/how-to-repair-the-efi-bootloader-on-a-gpt-hdd-for-windows-7-8-8-1-and-10-on-your-dell-pc?lang=en

This article provides information about how to resolve an issue with the EFI Bootloader not booting correctly on a GPT Hard Disk Drive for a number of Windows Operating Systems.

Table of Contents:
Introduction
How to repair your EFI Bootloader
How to repair your MBR (Master Boot Record)

From: https://blog.d0zingcat.xyz/2015/09/28/Windows/How%20to%20repair%20the%20EFI%20Bootloader%20in%20Windows%2010/

NOTE: have succesfully used Repair on Windows 2018 version install disk.

The Fix
1. Assign the Drive Letter
2. Repair the Boot Record
3. Rebuild the BCD store

From: https://forums.tomshardware.com/threads/windows-10-wont-boot-bootmgr-is-missing-bootrec-fixboot-access-denied.3297648/

Here are commands to fix bootloader:
diskpart
select disk 0
select partition 2
assign letter=g
exit
bcdboot c:\windows /s g:

From: https://pureinfotech.com/restore-registry-backup-windows-10/

Type the following command to move inside the RegBack, which contains a 
backup of the Registry, and press Enter:

cd c;\windows\system32\config\RegBack

http://msdn.microsoft.com/en-us/library/windows/hardware/gg463059.aspx

Microsoft has completely reengineered the boot environment for Microsoft Windows Vista to address the increasing complexity and diversity of modern hardware and firmware.

http://www.pronetworks.org/forums/how-to-edit-the-windows-vista-boot-menu-options-t79102.html

  bcdedit /set {5189b25c-5558-4bf2-bca4-289b11bd29e2} description "Windows Vista Build 5270 x64"

Changes the text of the boot menu line for any other Vista installation. One must use the GUID for that particular installation as shown when one runs the bcdedit or bcdedit /enum all command

Correcting changes to the Partition/Disk structure

Where a partition or a hard drive has been added or removed and has caused the partition/disk structure to change, this can be corrected by running these commands in the order shown:

  X:\>X:\boot\fixntfs.exe -lh -all (Where X: is the drive/partition on which the folder "boot" is to be found)

  bcdedit /set {5189b25c-5558-4bf2-bca4-289b11bd29e2} device partition=X:

Changes boot partition of the OS whose GUID is indicated. (Where X: is new drive/partition required). Must be used together with the osdevice command below

  bcdedit /set {5189b25c-5558-4bf2-bca4-289b11bd29e2} osdevice partition=X:

Changes boot partition of the OS whose GUID is indicated. (Where X: is new drive/partition required). Must be used together with the device command above

From: https://technet.microsoft.com/en-us/library/hh825146.aspx

====== This documentation is archived and is not being maintained. Sample: Configure BIOS/MBR-Based Hard Disk Partitions by Using Windows PE and DiskPart mbr recovery partition requirements to boot

From: https://technet.microsoft.com/en-us/library/hh825677.aspx

From: https://technet.microsoft.com/en-us/library/hh825089.aspx

From: https://msdn.microsoft.com/en-us/windows/hardware/commercialize/manufacture/desktop/configure-biosmbr-based-hard-drive-partitions

From: http://www.partition-recovery.com/

Active@ Partition Recovery is a freeware toolkit that helps you to recover deleted and damaged logical drives and partitions within DOS, Windows, WinPE (recovery boot disk) & Linux (recovery LiveCD) environments.