Windows 7 Support Center

Upon purposely changing the active partition on my system drive, I was faced with a BOOTMGR is missing error during my system's start up that prevent Windows from starting. It is a common mistake to make when playing with partitions on a system drive and it can be a headache to solve if not prepared. To change your active partition back using the Windows 7 recovery disc or Installation DVD, follow the steps below.

Step one: Follow steps one to four in the above guide. This should take you to the Command Prompt in the Windows Recovery Environment.

Step two: Type DiskPart and then press Enter.

Step three: Type List Disk now and then press Enter. This command will list all disks attached to your computer and assign them a disk number.

Step four: Type Select Disk x, where x is the number for the disk containing the partition you wish to make active. Press Enter.

Step five: Type List Partition and then press Enter. You will now be shown a list of the partitions on the selected disk. Determine which partition you wish to make active.

Step six: Type Select Partition x, where x is the number of the partition you wish to make active.

Step seven: Now, just type Active and then press Enter. That should be it - the selected partition is now active.