What Is a Partition?

Disk partitions: What they are and how they work

A disk partition can be thought of as a division or "part" of a real hard disk drive. Really, it's only a logical separation from the whole drive, but it appears as though the division creates multiple physical drives.

Some terms you'll see associated with a partition include primary, active, extended, and logical partitions (more on this below). Partitions are also sometimes called disk partitions, and when someone uses the word drive, they usually mean a partition with a drive letter assigned.

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How to Partition a Hard Drive

In Windows, basic hard drive partitioning is done via the Disk Management tool. See How to Partition a Hard Drive in Windows for detailed steps on creating a partition in each version of Windows.

Advanced partition management, like expanding and shrinking partitions, joining partitions, etc., can't be done in Windows but can be done with special partition management software. We keep updated reviews of these tools in our Free Disk Partition Software Tools list.

Keep reading to learn more about why you might build partitions and to understand the different types of partitions that can be created.

Benefits of Using a Partition

Dividing a hard drive into partitions is helpful for a number of reasons, but is necessary for at least one: to make the drive available to an operating system.

For example, when you install an operating system like Windows, part of the process is to define a partition on the hard drive. This partition serves to define an area of the hard drive Windows can use to install all its files, from the root directory on down. In Windows, this primary partition is usually assigned the drive letter of "C".

In addition to the C drive, Windows often automatically builds other partitions during installation, even though they rarely get a drive letter. For example, a recovery partition, with a set of tools called Advanced Startup Options, is installed so you can fix problems that might occur on the main C drive.

Another common reason to create a partition is so you can install multiple operating systems on the same hard drive, allowing you to select which one you want to start, a situation called dual booting. You might run Windows and Linux, or Windows 11 and Windows 10, or even three or four different operating systems.

Unless you use a virtual machine, more than one partition is an absolute necessity for running more than one OS because the operating systems will view the partitions as separate drives, avoiding most issues with each other. Multiple partitions lets you avoid having to install multiple hard drives just to have the option of booting to a different OS.

Hard drive partitions might also be created to help manage files. Even though the different partitions still all exist on the same physical drive, it's often useful to have a partition made up just for photos, videos, or software downloads instead of storing them in separate folders within the same partition.

While less common these days, thanks to better user management features in Windows, multiple partitions could also be used to help support users that share a computer and would like to keep files separate and easily share them with one another.

Another, relatively common reason you might create a partition is to separate the operating system files from personal data (e.g., home movies or a music collection). With your valuable, personal files on a different drive, you can reinstall Windows after a major crash and never get close to the data you want to keep.

This example also makes it really easy to create a mirror image backup of a working copy of your system partition. This means you could build two separate backups, one for your in-working-order operating system, and another for your personal data, that can each be restored independently of the other.

Primary, Extended, and Logical Partitions

Any partition that has an OS installed to it is called a primary partition. The partition table portion of a master boot record allows for up to four primary partitions on a single hard drive.

While four primary partitions can exist, which means a total of four operating systems could be quad-booted on the same drive, only one of them is allowed to be "active" at a time, which means it's the default OS the computer boots to. This partition is referred to as the active partition.

One (and only one) of four primary partitions can be designated as an extended partition. This means a computer can have up to four primary partitions or three primary partitions, and one extended partition. An extended partition can't hold data in and of itself. Instead, it's simply the name used to describe a container that holds other partitions that do hold data, which are called logical partitions.

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There's no limit to the number of logical partitions a disk can contain, but they are limited only to user data, not operating systems like with a primary partition. A logical partition is what you'd create to store things like movies, software, etc.

For example, a hard drive will generally have a primary, active partition with Windows installed on it, and then one or more logical partitions with other files like documents, videos, and personal data. Obviously, this will differ from computer to computer.

More Information on Partitions

Partitions of physical hard drives must be formatted, and a file system must be set up (which is a process of the format) before any data can be saved to them.

Because partitions appear as a unique drive, they can each be assigned their own drive letter, such as C for the partition Windows is usually installed to. See How to Change a Drive Letter for help doing this in Windows.

Normally, when a file is moved from one folder to another under the same partition, it's just the reference to the file's location that changes, meaning the file transfer happens nearly instantaneously. However, because partitions are separate from each other, like multiple hard drives, moving files from one partition to another requires the actual data to be moved, and will take more time to transfer the data.

Partitions can be hidden, encrypted, and password protected with free disk encryption software.

  • How do you merge disk partitions?

    To merge two partitions, open the Disk Management tool (Windows+x > Disk Management), right-click the drive you want to delete, and select Delete Volume to change the disk space to Unallocated. Next, right-click the drive you want to extend, select Extend Volume, and follow instructions.

  • What is the maximum partition size NTFS supports on a dynamic disk?

    The maximum partition size of NTFS depends on the smallest cluster size. By default, NTFS can support hard drives up to just under 16 EB, and individual files at just under 256 TB.

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