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In the disk management If I right click on the C: drive there is a option Mark a partition as active.

What happens when I mark my C: drive partition as active ?

4 Answers 4

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The partition that has Windows on it is already marked active.

If you mark another partition active, it will mark all other partitions as not active. If you reboot your computer with the Windows partition mark not active, it will not boot into Windows; it will probably say OS not found.

Unless you feel comfortable with the partitioning, formatting and installing files, I suggest that you leave the modifications to someone with more experience.

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  • but I don't see this option on other drives except C drive.
    – A. Prasad
    Aug 13, 2014 at 6:00
  • You can only have one partition active and it must be a 'primary partition', the partition without that option will be an 'extended' one.
    – edumgui
    Aug 13, 2014 at 12:30
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Be careful to active partition. On MBR disk, the system reserved partition is required to be active. If you mark C: active, Windows will be unbootable. On GPT disk, the Mark Partition as Active option in Disk Management is grayed out. You can search accidentally marked drive C: active in google to find more details.

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Unless you are familiar with what you're doing- you would be well advised not to play around with the active status of various disk partitions.

E.g. UEFI and other considerations have to be taken into account- thus- if you look at Windows 10 on a modern computer- it'll have a small (usually 100Mb) hidden partition marked as active- whereas the main 'c:' drive- contrary to what you expect- is not the active partition- and if you modify it to active status- when you reboot you'll get an error message 'Unable to load MBR'.

Honestly- if you are not happy to play around with partitions, system files- and if you screw up- system prompt instructions for modifying structures (or using a Linux distro installation disk to boot- so you can modify the partition information back again)- just don't do it.

Also- your 'c:' drive- or the drive on which you have your windows folder- contrary to what a lot of people will tell you- does not have to be active- your partition on which your MBR is- does. They can be the same partition (and usually are esp in some older operating systems)- but do not have to be- and you need a reasonable level of competence to figure what you're doing.

I would strongly advise you do a little research of UEFI MBR GPT partitions. The Microsoft knowledgebase have some decent articles.

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    UEFI has no concept of an active partition. UEFI machines have their own partition scheme (GPT), which does contain a "protective" MBR, but the active bit in the record is ignored and the OS to be booted is determined by variables in nvram and the contents of the EFI System Partition. Mar 19, 2018 at 16:12
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You tell the Windows Boot Loader to load the Boot Manager from C drive.

The C drive may or may not contain the Boot Manager. If it does not, you'll get an error on boot.

If the system loads successfully, the C drive will be also marked as "System". In other words, "Active" is a potential boot partition, and "System" is the actual current boot partition.

The role of a Boot Manager is to find the OS files.

The role of the Boot Loader is to take over loading after you power on the machine - it is the first thing that the computer loads.

A boot loader may also act as a Boot Manager, as is the case with GRUB; with the Windows Boot Manager, they are separate; both can load multiple operating systems, however.

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