How to Mark a Partition as Bootable in Windowsby Contributor ; Updated September 28, 2017
The partition that boots your computer system has to be a primary partition that is active for your system to be bootable. Only the partition on which the operating system is stored should be marked as active, since only one partition can be active at a time. The operating system will not load if its partition is not active, thus rendering your computer un-bootable. Fortunately, in Windows, you can use the Disk Management utility to repartition, format and mark hard drives as active for booting.
Click "Start," "Control Panel," "System and Security" and "Administrative Tools." Double-click "Computer Management."
Click "Disk Management" in the left pane of the Computer Management window.
Right-click the partition you want to make bootable. Click "Mark Partition as Active." Click "Yes" to confirm. The partition should now be bootable.