What exactly is an active partition?

Dennis P December 22, 2013
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I have read about ‘active’ partitions on the question board here and they stated that they were partitions that could be booted from, but I’ve got a couple of questions…

  1. I’ve got Windows Vista loaded on both my SSD (drive C:, marked “Active”), yet I’ve got Vista loaded on my internal HDD too (drive D:) and it isn’t marked ‘active’, so the question is, why isn’t drive D: marked ‘active’?, plus…
  2. I’ve got a 1TB external USB HDD (Western Digital – My Book) that has two partitions on it, the first one is drive G:\Images and has Macrium Reflect images stored on it and it is marked ‘active’. The other partition is drive H:\Storage and it just for miscellaneous file storage and it isn’t marked ‘active’. Why is drive H:\Images marked ‘active’ when Windows isn’t loaded on it?

This is confusing me. Can anyone explain this to me? I would appreciate it.

  1. Oron J
    December 22, 2013 at 3:25 pm

    Regarding your first question, every drive can have up to four partitions, one of which could be marked as "Active". When your system boots from a particular drive, it loads the code from the active partition of that drive. Ordinarily this means you can boot from one system on each hard-drive, provided the drive is bootable at all. It is possible to set up multiboot setups, but this requires either installing the systems in a very specific order, or learning more about the boot process and customising a boot-loader, which is a special application which will "boot" first and allow you to choose the system you want to start from.

    As for the second question, I hope the answer above addressed it. Every hard drive can have one active partitition, but this is only relevant if it is actually used for booting up the computer.

  2. Hovsep A
    December 22, 2013 at 2:21 pm

    your BIOS is set to boot on SSD thats why SSd is active and not D
    Mark a partition as active

    I Think Hard drive images include hard drive marked as an active disk since it is the Windows image with MBR,

    • Dennis P
      December 22, 2013 at 5:22 pm

      I messed up on item number 2., as I said "Why is drive H:Images marked ‘active’ when Windows isn’t loaded on it?" and it should have read "Why is drive H:Storage marked ‘active’ when Windows isn’t loaded on it?".

      So you think that drive "H:Images" is marked active because the system found an image with a MBR in it?

      Thanks so much for your help in this matter...

    • Bruce E
      December 22, 2013 at 6:51 pm

      1. The only MBR that matters is the one in the very first sector on the drive in question, not any system images that may be stored on it.
      2. Any hard drive can have one partition marked as active whether there is an operating system installed on it or not. Preformatted external drives will frequently have only a single partition, but it is occasionally marked as active.
      3. Marking a partition as active means that the bootloader (in the MBR) will look in that partition for the code required to continue booting the machiine. The code in the active partition will be the portion that would present alternate OS boot options on a multi-boot system so you may have partitions with other operating systems on them that are not marked as active even though they are the partitions where the OS code for one of your boot options lives.

    • Hovsep A
      December 22, 2013 at 9:17 pm

      Are you having problems when plugging your external hard drive to Windows pc with the partition marked as active?

      Did you made the two partitions on the external hard drive yourself and marked one of them as active before using Macrium Reflect. Or your Macrium Reflect did that and set that partition as possible bootable so set Active Flag? 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 PC.

      Each hard drive also has one of its possible 4 partitions flagged as an active partition. A primary partition is in which an Operating System can be installed. There is also a hidden 100mb/350mb NTFS partition (for instance if you install windwos 7, 8...) namely boot partition storing boot files of your operating system, this partition is set to be active partition by default. For Vista i think the boot files were in the operating system partition so this made easier for users to make mistakes and malwares to do damage. The active partition is assigned to only one partition on a hard drive that the Master Boot Record uses to boot your pc into an operating system, But since your external hard drive contain an image (no real operating system) you do not need a bootloader to load a particular OS. If you do not like the Active flag perhaps you can change it.

      Before Windows 7, the system and boot partitions were, by default, the same and were given the identifier "C:". After Windows 7, however, Windows Setup creates a separate system partition that is not given an identifier and therefore is hidden. The boot partition is still given "C:" as its identifier.

      To mark a partition as active

      Read Bruce E and Oron J comments for detailed explanation.