How To Change Active Partitions

DiskPart

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

STEP 2: Type DiskPart and then press Enter.

STEP 3: 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 4: Type Select Disk x, where x is the number for the disk containing the partition you wish to make active. Press Enter.

STEP 5: 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.
You’ll see a list which will display your system partition.
(this is an example list)

Partition 1 OEM 39 MB
Partition 2 Primary 750 MB
Partition 3 Primary 297 GB < select the partition with Windows, which is probably the first one larger than 16 GB

STEP 6: Type Select Partition x, where x is the number of the partition you wish to make active.

STEP 7: Now, just type Active and then press Enter. That should be it – the selected partition is now active.