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Here Are 3 Solutions after Accidentally Marking Drive C Active

  • Contents:

    Note: this post is much related to MBR disk rather than GPT disk.

    Several months ago, one of my friends told me that his Windows operating system cannot boot after he accidentally marked drive C active, and I finally helped him fix the issue after a series of researches. Several days ago, when I was browsing a famous Windows related forum, the same question was found, and till now it hasn't been resolved. Therefore, I plan to write this post, hoping it useful and helpful for users who are troubled by the same issue.

    My post contains 3 parts, including which partition should be set active, why this partition should be set active, and how to set the correct partition active. Next, let's see these parts one by one.

    Which Partition Should Be Set Active

    Before answering this question, we need to know something about system partition and boot partition, because both of them play significant roles in booting Windows. As loading architecture of Windows OS changed a lot since Windows Vista, here we just take Windows Vista for example. In Vista:

    System partition is the partition mainly holding: 1. Windows boot manager (BOOTMGR) which replaces NTLDR used on earlier Windows OS like Windows XP and Windows Server 2003; 2. the boot configuration database (BCD) which replaces original boot.ini; 3. Windows recovery environment. It is marked as System in both Disk Management and MiniTool Partition Wizard.

    Boot partition is the partition saving files for Windows operating system (usually \WINNT), and it is marked as Boot.

    system and boot partition in partition wizard

    accidentally marked drive c active 1

    In old versions of Windows OS, system partition and boot partition are contained in the same partition, and it is always the drive C.

    system and boot partition are contained in one drive

    However, if more than 1 operating system is installed, there can be more than 1 system partition or boot partition. For example, if we install Windows XP on drive C and Windows Vista on drive D, both C and D can be called boot partition or system partition.

    But since Windows 7, system partition and boot partition can be either separate or merged. If we make a Windows installation on a never-used hard disk (the disk having no partition), these 2 partitions will be separate. The partition which is marked as system reserved (100MB for Windows 7 and 350MB for Windows 8) in Disk Management is the very system partition while drive C now is only a boot partition.

    Once a system reserved partition is created, there can only be one system partition on this disk no matter how many operating systems are installed, because boot programs of all installed Windows operating systems will be saved to this partition automatically. However, there may be multiple boot partitions. Let's see a specific example.

    Firstly, we install Windows 7 to create a hidden system reserved partition as well as a boot partition (drive C), and then install Windows 8 to drive D. At this time, system reserved partition is the unique system partition while drive C and drive D are boot partitions. However, if we install Windows to a hard disk where partitions have been created, system partition and boot partition will be contained in one partition like the old versions of Windows OS.

    Summary: partitions saving Windows boot programs like BOOTMGR and BCD or NTLDR and boot.ini is called system partition, not limited to drive C. That is to say sometimes it is the system reserved partition, and sometimes it is the drive C. And generally, system partition should be set active to make Windows boot successfully. If there are multiple system partitions on one disk, which means there are equal Windows operating systems, we need to set the needed one active. Well then, why should we set system partition rather than any other partition like boot partition active? Let's see the specific reasons.

    Why System Partition Should Be Set Active

    If system partition is not set active, Windows will be unbootable. Why? To get the answer, let's see how Windows is loaded on an MBR disk.

    Once we switch on the power button of the computer, booting process begins. After a series of Power-On-Self-Test (known as POST), BIOS will find and pass boot control to Master Boot Record (MBR). Then, MBR reads master partition table to find the primary partition which is marked as active, and next reads partition boot record of the active partition to locate Windows boot manager (BOOTMGR) and then passes boot control to BOOTMGR. BOOTMGR reads the file \boot\bcd (boot configuration database) 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.

    From the boot process above we know only setting the system partition active can MBR visit the very partition to find Windows boot programs BOOTMGR and Boot Configuration Database. If these boot programs cannot be located, Windows cannot be loaded, and errors like BOOTMGR Is Missing or NTLDR is missing may emerge.

    Therefore, when drive C is not the system partition but we accidentally marked it active, Windows consequently will be unbootable. Under this situation, to set the real system partition active can fix the issue, but how can we complete this change when Windows refuses to boot? Let's see solutions.

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    Here Are 3 Solutions after Accidentally Marking Drive C Active

    Method 1: Mark System Partition Active with MiniTool Partition Wizard

    MiniTool Partition Wizard could help users complete different types of partitioning operations, such as rebuild MBR, recover lost partition, set partition active/inactive, extend partition, migrate OS to SSD or HD, and copy hard disk/partition. If Windows is bootable, Partition Wizard Free is available. However, if you are unable to start Windows, Partition Wizard Bootable Edition is required, which is created by the built-in Bootable Media Builder of Partition Wizard.

    To set system partition active when there is bootable Windows available, please start this bootable Windows (you may need to set boot order in BIOS for successful boot), download MiniTool Partition Wizard Free Edition to your computer, and follow steps below.

    Launch MiniTool Partition Wizard to get its main window:

    set boot partition inactive when a bootalbe system is available

    Select the boot partition of the unbootable Windows and mark it inactive via the "Set Inactive" feature. Then, select the system partition of the unbootable Windows and mark it active via the "Set Active" feature.

    Tip: after booting from another Windows, you may find letter C has been allocated to another boot partition rather than the previous one.

    At last, click "Apply" button to execute changes.

    After setting correct boot order in BIOS, users should be able to boot Windows.

    Well, what if there is no bootable Windows available on this computer? To mark system partition active under such a circumstance, users need to purchase a license of Partition Wizard and then create a bootable disk using the built-in Bootable Media Builder on another computer. Purchase it here with enjoying discounts.

    After successfully registering the program, you’ll get the following window:

    mark system partition active when Windows is unbootable create bootable disk

    Here, please choose Bootable Media icon at the top right to launch the Bootable Media Builder.

    Then, create a bootable CD, DVD, or USB flash drive. For detailed steps, please refer to How to Build Boot CD/DVD Discs and Boot Flash Drive with Bootable Media Builder.

    Once the bootable disc is created, please boot computer from it. For steps to boot from a burned disc, please refer to Boot from MiniTool Bootable CD/DVD Discs or USB Flash Drive.

    After successful boot, you’ll get the following window:

    mark system partition active when Windows is unbootable set wrong partition inactive

    Here, please select the partition that was mistakenly marked as active and set it inactive.

    Tip: in normal Windows, the system partition may not have a letter while in MiniTool Partition Wizard Bootable Edition it has.

    mark system partition active when Windows is unbootable set system active

    Then, select the system partition and set it active.

    At last, click "Apply" button to perform changes.

    After that, Windows should be bootable as long as users set to boot from the correct device in BIOS.

    Then, let's see the second way.

    Method 2.Mark Correct Partition Active with Windows PE

    Firstly, we need to create a Windows PE bootable CD, DVD, or USB flash drive. Please see WinPE: Create a Boot CD, DVD, ISO, or VHD and WinPE: Create USB Bootable drive of Microsoft to get detailed steps. If you have such a bootable disc, just ignore this step.

    Then, boot computer via WinPE bootable disc and press a key to enter BIOS setup during boot process. Different computer requires different key, but there is always a prompt like this:

    press a key to enter BIOS setup

    Tip: there are only few seconds to make this operation. If you missed this chance unluckily, just reboot computer again. After entering BIOS successfully, we need to set boot order, namely setting the CD-ROM (bootable CD or DVD disc) or Removable Device (bootable USB flash drive) as the first boot device by making use of keys like "↑", "↓", and "Enter".

    set first boot device

    After the setting is done, we need to press on "F10" to save changes and exit from BIOS setup utility, and then our computer will boot from the Windows PE bootable disc. After Windows PE starts successfully, we can see the desktop:

    start diskpart in windows PE

    Now we need to run Diskpart, which is a Windows built-in powerful partitioning tool. Type diskpart in searching box, right click the program and run it as administrator. Tip: if we do not run it as a administrator, partitioning operations may be not allowed. After successful running, we can see the following window:

    set drive c inactive set system partition active

    Here we need to set drive C inactive at first, and then mark the system partition active. Detailed steps are as follows:
    • Type list disk and press on Enter.
    • Type select disk n (n is the number of the disk you will operate) and press on Enter.
    • Type list partition and press on Enter.
    • Type select partition n (now n is the number of drive C) and press on Enter.
    • Type inactive and press on Enter.
    • Type select partition n (n is the number of the correct system partition) and press on Enter.
    • Type active and press on Enter.
    • Type exit and press on Enter to safely quit from diskpart.

    After doing these operations, we should be able to boot our Windows if we set the original hard disk as the first boot device in BIOS in advance.

    However, if you think it difficult to create a Windows PE bootable disc and there is a Windows installation media available, which could be CD, DVD, and USB flash drive, take the third solution.

    Method 3.Mark Active Partition Correctly with Windows Installation Disc

    First of all, we also need to boot computer via the bootable disc and then set CD-ROM or removable device as the first boot device in BIOS, and detailed operations have been introduced in Method 2. When the language screen appears, please make use of key combination Shift + F10 to call CMD out, and then enter diskpart to invoke the program.

    language screen of windows installation disk

    Since steps to mark partition active and inactive with diskpart have been laid out in solution 2, here we ignore this issue. Apart from diskpart, users can also try employing system repair function in Windows installation disc to fix Windows boot errors. In the language screen just click "Next" button at the lower right corner to get the window below:

    repair computer in windows installation disc

    Here please choose "Repair your computer". Then, select the specified Windows to repair and click "Startup Repair".

    choose startup repair

    After successful repair, please reboot computer to see whether Windows can be loaded normally. Tip: this function can only help fix some specified boot problems, for example system files get lost, so this cannot 100% fix the issue.


    If your Windows OS also cannot boot after marking active partition mistakenly, try one of these solutions out in accordance with actual demands. Actually, accidentally marking drive C active is just one of factors which may make Windows unbootable, and the damage of MBR or the loss of boot partition and system partition can also result in the same error. "Partition Recovery" feature and "Rebuild MBR" in MiniTool Partition Wizard Bootable CD are capable of fixing these problems.