Even if a partition contains bootable data, your computer might sometimes fail to detect it as a suitable bootable partition. This is usually due to the fact that, to be recognized by your computer as a bootable partition, a partition does not only need to contain a bootable environment, it also needs to be specifically marked as an active system partition. You can add that active marker to a hard drive partition by using Diskpart, a command-line disk management application that is built into Windows.
Open the Windows Start screen and type "cmd." Right-click on the "Command-Line Prompt" shortcut and select "Run as administrator."
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Type "diskpart" and press the "Return" key. Diskpart will now launch. While Diskpart is active, the traditional command-line prompt will be replaced by a Diskpart prompt, indicating that any command you enter will be processed by that application.
Enter "list disk" and press the "Return" key. Note the number assigned to the drive which contains the partition you wish to make bootable. You can find that number in the first column.
Type "select disk X", replacing "X" with the number assigned to the relevant drive. Press the "Enter" key.
Enter "list partition" and press the "Return" key. Note the number assigned to the partition you want to make bootable.
Type "select partition X", replacing "X" with the number of the partition you want to select. Press the "Enter" key.
Enter "active" and press the "Return" key to instruct Diskpart to mark that partition as an active, bootable partition.
Type "exit," followed by the "Return" key, to exit Diskpart. Close the Command-Line Prompt window.
By itself, marking a partition as active might not mean that your computer will automatically boot from it the next time it restarts. To boot from the partition you marked as active, your computer needs to find a suitable bootable environment on it, such as an operating system or a recovery application. In addition, you might have to modify your computer's boot order in the BIOS to prevent it from booting from other active partitions that might be higher in the boot order. To do so, please refer to your motherboard's instruction manual.
Only one active partition can be present on any one hard drive.