Active partition is a primary partition on a disk from which the Windows will load all its files, means that is set as bootable. Only the partition on which operating system has been marked as active, hence an active partition is always a startup partition. If there are several OS's on one disk, Windows will load from the one marked as active.
Need for Active partition
Hard drives can be partitioned in various ways for managing and organizing data. If you want your drive to be bootable, it should be partitioned as the "Active" partition. Marking a partition as active on a basic disk means that the computer will use the loader (an operating system tool) on that partition to start the operating system.
For instance, if you are using Microsoft Windows, the partition that contains Windows OS is the active partition. If you create a partition and mark it active when it doesn't have an operating system boot loader on it then your computer doesn’t boot.
Points to remember before marking active partition:
How to Mark a Partition as Active?
Note: Before marking a partition as “Active”, you must be logged in as an administrator
Difference between Primary and Active Partition
At the time of installation, Windows creates one primary partition that comprehends entire disk. But you can create up to four primary partitions on your computer. In case you want more than one OS on your computer (multi-boot system) you have to create one primary partition for each operating system. You may even utilize additional partitions in order to manage, organize, and separate your files. Doing so often improves system performance and nothing will happen to your data if the OS is unbootable, corrupted, or infected with malware/spyware.
Whichever primary partition from which you load your operating system at the time of booting becomes active partition. You may choose only one partition active at a time. However, if you have multiple operating systems then you have to mark that specific operating system's partition as active.
If you disable the Disk Defragmenter service, you will receive an error message when you try to do anything in Disk Management. This indicates that the ‘Disk Defragmenter’ service is set to only manual.
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