Multibooters.com

Change the Active Partition
With Microsoft's Disk Management Tool.

Not to be done lightly, but if used correctly it can act like a bootmanager,
or determine which partition the boot files will go to during a Windows install.

For a primer and details on how to start the Disk Management tool please refer to the first article in this series.

The warning dialog that is displayed when you click to change the Active partition is not exaggerating and in fact understates the issue by failing to mention that if you have a dual or multiboot system that has been natively configured to use Windows' own bootmanager, then changing the active partition will disable that bootmanager. This will most likely leave you unable to boot anything, regardless of the fact that the newly set active partition may indeed contain a 'functioning operating system'.

Selecting Yes to continue past the warning box will see the operation completed in a matter of seconds and the Active label will be moved (fig:3). Under normal circumstances the Active and System partitions are always one and the same because the Active tag is simply a pointer to direct the boot sequence to the location of the next files in the boot process, which as they load will make the partition they are on the System partition. If we now reboot the computer our new Active partition will, if it contains the correct components, become the new System partition.

As you can see in fig:4 we have rebooted into the Vista operating system on the first partition and it is indeed now both the Active and the System partition. It has also become the Boot partition, and this is our main clue that this is now the partition and the operating system that we are booted into. When the Windows bootmanager has not been configured for use then an operating system will typically be both the System and Boot partition, because the boot files will be on the same partition as the operating system. When the Windows bootmanager (or the Win-7/8 System Reserved partition) has separated boot-files and operating system to different partitions then the System and Boot partitions may be different, which by consequence means that dependent Boot partitions won't have their own boot-files and so will forever be reliant on their System partition. All of which explains why moving the Active status to a dependent Boot partition will result in an unbootable machine. The Windows System and Boot partitions.  The Windows7 System Reserved Partition.

The other situation where we might want to manually change the Active partition is when we are doing a Windows install where we don't want the default Windows bootmanager being automatically configured for us. If we pre-make the partition for a new install and set it as the Active one on the disk, then a Windows install that is run from the DVD or other boot media and directed to use this Active partition will put all of its boot files on it, thereby making it fully self-contained and not linked to or dependant on any of our other partitions or operating systems. This gives as the freedom to use a different bootmanager, or of course we can still use the Windows bootmanager by configuring it ourselves, but our Windows installs will retain the capacity to be independent, which can make for easier and safer cloning, imaging and moving of operating systems.

Here we have created a new but blank partition on the drive and set it as Active (fig:5). If we were to now try and reboot from the hard drive no operating system would be found, so we have to be ready at this point to start our new install and be booting from our install media. Our partition has been left unformatted (Raw) as we can get the Windows install to do that, but we could just as easily install to a pre-formatted partition. After our new system is up and running we can reinstall or reconfigure our bootmanager to again give us access to all our other operating systems, or we could just change the Active partition to the operating system we want to boot into. If our new install of Windows had failed for some reason and we became unable to get to Disk Management to reset the Active partition then we would do it from one of our recovery tools. For more on preventing the auto setup of the Windows bootmanager see our guide Installing Windows to be independent.

Only Boot Drive Primary Partitions.
The Active tag is mainly used by the MBR to determine which partition contains the next required files in a boot sequence, so it is only the boot hard drive where the presence of a partition marked as active has any relevance or use. In Windows it is only primary partitions that can natively be marked as active and any logical partitions or active partitions on other drives or devices are of no consequence to Windows.

Windows can function quite happily without the presence of any active partitions at all, which can happen when it has been booted in a way that has bypassed the normal MBR boot sequence, such as from a boot disk or some third-party bootmanagers. There is one instance where Windows does reference the active partition and it can cause a few issues for us if we have strayed from the native Microsoft configuration where the Active and System partitions are always one and the same. If we have no boot drive active partition or if our system and active partitions are not the same then we can have problems using hibernation, or altering options of the Windows bootmanager. For more on this see... Windows Hibernation and the BCD.