Changing the active partition is a fairly advanced task, so if you
are planning on doing this, you probably didn’t need the explanation
above! Only change the active partition if there is an operating system
residing on that partition, otherwise you will have a non-working
computer.
Also, there are a few things to note about marking a partition as active:
- A logical drive or extended partition cannot be marked as active, only primary partitions can be changed to active.
- You can only have one active partition per physical hard disk. Trying to do otherwise will cause all kinds of problems.
- If you have several physical hard disks on your computer, you can
mark a partition as active on each disk, but only the active partition
on the first hard disk detected by your BIOS will start up the computer.
You can go into the BIOS and change the order to detect hard disks.
In addition to the above notes, there are additional things that have
to be in place in order for the system to boot. Just setting a
partition to active does not ensure that the system will boot properly.
- The active partition must have a boot sector that was created by the operating system
- The active partition should contain the boot loader and boot files
for the OS and must know the location of the OS on the physical hard
drive
- The actual operating system has to be located in the correct spot on the hard drive
When the computer boots up, it will look for an active partition on
the primary partitions first. The boot sector, located at the beginning
of the active partition, will run the boot loader, which knows the
location of the operating system boot files. At this point, the
operating system will boot up and run.
Set Active Partition via Disk Management
Open Computer Management by going to the
Control Panel, clicking on
System and Maintenance, and then clicking
Administrative Tools.

You can also click
Classic View and then choose
Administrative Tools. Now click on
Disk Management under
Storage.

Another option is to go to your desktop, right-click on
Computer or
This PC and choose
Manage.
You’ll see Disk Management in the left hand menu like shown
above. Right-click on the primary partition that you want to mark as
active and choose
Mark Partition as Active.
Set Active Partition via Command Line
If you screwed something up in Windows and marked the wrong partition
as active, you will no longer be able to boot your computer. In the
case where you cannot mark a partition as active using Windows, you’ll
have to us the command line.
Depending on your version of Windows, getting to the command line can be a bit tricky. Read my post on
restarting Windows in safe mode to get to the system repair options for Windows XP, Vista, 7, 8 and 10. If you’re running Windows 8, read my post on
booting to system recovery options. Once there, you have to go to
Troubleshoot, then
Advanced Options, and then click on
Command Prompt.

First get into the command prompt using a boot up disk and type
diskpart at the prompt.

At the DiskPart prompt, type in
list disk. You will see the list of disk currently attached to your computer. Now type in
select disk n, where n is the disk number. In my example, I would type
select disk 0.

Now that we have selected the correct disk, type in
list partition to get a list of all the partitions on that disk. To select the partition we want to set as active, type in
select partition n, where n is the partition number.

Now that we have selected the disk and partition, we can mark it as active by just typing the word
active and pressing Enter. That’s it! Now the partition is set.
Most people are familiar with using FDISK to mark a partition as
active, but that is now an older and outdated command. You should use
DISKPART to manage disk and partitions on a modern Windows computer.
Set Active Partition via MSCONFIG
In addition to the above two methods, you can use MSCONFIG to set the
active partition. Note that this method only works for primary
partitions on the same hard drive as MSCONFIG won’t detect partitions on
other hard drives. Also, the other partition can only have Windows
installed in order to set it as active.
If you’re not familiar with the MSCONFIG tool, read my previous
guide on using MSCONFIG. Open MSCONFIG and click on the
Boot tab.

You’ll see the operating systems listed and the active one will have
Current OS; Default OS after the name of the operating system. Click on the other operating system and then click on
Set as default.
There are other methods to set an active partition like using a Linux
live CD, but these are much more complicated and normally not
necessary. Even if the built-in system recovery options don’t allow you
to get to the command prompt, you can always use a secondary computer to
create a bootable USB system recovery drive.