You would feel helpless and desperate when Windows can’t boot due to mistakenly deleting system partition, Actually, there are lots of other folks facing similar issues, And the error message you’ll receive when encountering an unbootable Windows varies; one of the most common messages is – operating system not found
So, This guide we will show why Windows won’t start after system partition is lost as well as how to make Windows bootable again.
When you are installing Windows 7/8/8.1/10 on a clean disk which is initialized to MBR, the setup will create a system reserved partition at the beginning of the hard disk and mark it active. It does not have a letter, so you can’t find it in Windows Explorer or Computer but only visible in Windows Disk Management or other special tools. This partition holds the Boot Configuration Database (BCD), Boot Manager (BOOTMGR), and Windows Recovery Environment and reserves space for the startup files which may be required by BitLocker, among which BCD and BOOTMGR are vital parts to start Windows.
Then, let’s see how Windows is loaded and running.
After we power on the computer, BIOS will perform Power-On-Self-Test (POST). When the test is completed, BIOS will look for and pass boot control to MBR (Master Boot Record). Then, MBR reads master partition table to find the active partition, namely the system reserved partition, and reads partition boot record of the system partition to locate and give boot control to Boot Manager. Boot manager then reads BCD which contains the menu entries that are presented by the BOOTMGR, and these entries can be: Options to boot Windows by invoking winload.exe, Options to resume Windows from hibernation by invoking winresume.exe, etc. If there are multiple Windows OS, we are able to choose which Windows to boot. After a Windows OS is selected, BOOTMGR will invoke winload.exe to load the corresponding Windows kernel as well as the core device drivers so as to boot Windows.
You might feel a little bit difficult to get an understanding of the description above, so below we give a flow chart showing how Windows is loaded.
If the active system reserved partition is lost, Windows will be unbootable since no BOOTMGR and BCD are found. And you may receive error messages, like BOOTMGR is missing or Operating System not found.
What Is EFI System Partition (UEFI + GPT)
When you install Windows on a clean disk that is initialised to GPT, Setup will create an EFI partition (aka ESP). It contains the boot loaders or kernel images for all installed operating systems which are contained in other partitions on the same or any other local storage device, device driver files for hardware devices present in a computer and used by the firmware at boot time, system utility programs that are intended to be run before an operating system is booted, and data files such as error logs.
When a computer is powered on and booted, UEFI firmware loads files stored on the EFI system partition to start installed operating systems and various utilities. If the EFI system partition is lost, Windows will be naturally unbootable due to missing boot files.
2 Tested Ways to Make Windows Bootable Again If You Deleted System Partition by Mistake
Here, we mainly introduce 3 ways to restore system: use free third party program, employ diskpart of Windows installation disc, and reinstall Windows without losing personal files.
Method 1: Fast Recover System Partition with MiniTool Partition Wizard Bootable Edition
MiniTool Partition Wizard Bootable Edition can help Windows users complete various partitioning operations when Windows is unbootable, including recover lost/deleted partition, rebuild MBR, resize system partition/boot partition, clone disk/partition, and so on.
To use this bootable partition manager to recover partition, you should purchase a paid version. If you are not a Server user, the Professional Edition is available. After you register the software, you can use the Bootable Media Builder to build the boot CD/DVD discs and boot flash drive.
Once the bootable disc is created, please start your computer from this disc.
After successful startup, you can see the window below:
Here, please select the disk where system partition was originally located and choose “Partition Recovery” function from the left side.
Then, set scanning range for the selected disk from 3 ranges and click “Next“. If you haven’t created new partition after partition loss, you can try choosing Unallocated Space since it costs less time. Otherwise, choose Full Disk.
Once partitions Partition Wizard detected are listed, please give check marks on all needed partitions and click “Finish” button.
Then, select the recovered system reserved partition and set it active. At last, click “Apply” button to make all changes finally performed.
Now, you can try starting computer from Windows side.
Method 2: Fix Boot Information with Windows Installation Disc
To use this method, please prepare a Windows installation disc which had better be the one you used to install Windows. Of course, the one holding the same version as your Windows is also available.
Then, you can start fixing.
However, since there are 2 types of system partitions, namely system reserved partition and EFI system partition, we will show operations separately.
For Legacy BIOS Users
Firstly, please boot computer from the installation media to get the interface below:
Then, press on Shift + F10 combination keys to call out Command Prompt, and type following characters.
In the window of Command Prompt, type diskpart to activate this partitioning tool. Then, type the following commands to set Windows OS partition (C drive most of the time) active:
Select disk #
(# is the number of the disk where your Windows is installed)
Select partition #
(here # is the number of the Windows OS partition rather than system reserved partition)
After setting the partition active, please type exit to quit from diskpart.
Then, in Command Prompt please type the following commands to fix boot information:
The bootrec /rebuilBCD command will search for Windows installations not included in the Boot Configuration Data (BCD) and then ask you if you’d like to add one or more to it. Just type Yes and then Exit to quit from Command Prompt.
Your Windows can’t boot after deleting system partition? Now, try one of our methods to make it bootable again.