Beginning from Windows 7 / Windows Server 2008 R2, when you install Windows with the default settings to an unformatted disk, apart from the system partition (disk C:\), an additional service partition with the size of 100 MB is created. It is called System Reserved. A letter is not assigned to this partition, and most users even doesn’t know anything about this hidden partition.
If on some reason System Reserved partition has been deleted, Windows won’t boot with the error “Operating System not found” since the boot partition with the bootloader configuration is missing.
On the installation screen, press Shift+F10 and run diskpart, then display the list of volumes:
As you can see, System Reserved is missing, and of course, the system cannot boot from the disk.
In this case, to restore your system, you need to recreate the bootloader configuration. But in order not to create the 100 MB System Reserved partition, the configuration file bootmgr can be recreated on a system drive C:\. To do it, in WinPe command run the following commands. Place the boot environment files on drive C:\ and specify the path to Windows directory:
bcdboot c:\windows /s c:
If successful, the command will return Boot files successfully created.
Then make C:\ partition active. To do it, execute the following commands (you may have different disk and partition numbers, be attentive):
select disk 0
select partition 1
Then run these commands one by one:
The second command will scan all disks and try to find installed Windows copies (Scanning all disk for Windows installations). In our example, the OS has been found in c:\Windows. The system will ask to add these Windows copies to the boot list (Add installation to boot list?). Approve it (Y).
Restart your computer and make sure that Windows is booted correctly. Make sure that bootmgr file is present in the root of drive C:\.
This manual also may be used to transfer Windows Bootloader configuration to different drive if the System Reserved partition is removed deliberately, when you want the system to be completely located on one partition (It is convenient from the point of view of some backup and disk management tools.)