Use Disk Utility to Add, Delete, and Resize Existing Volumes

Hand adjusting marble pie chart

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In the early days of the Mac, Apple supplied two different apps to take care of the day to day needs of managing a Mac's drives -- Drive Setup and Disk First Aid. With the advent of OS X, Disk Utility became the go-to app to take care of your disk needs. But aside from combining two apps into one and providing a more uniform interface, there was not a lot of new features for the user.

That changed with the release of OS X Leopard (10.5) which included a few notable features, specifically, the ability to add, delete, and resize hard drive partitions without first erasing the hard drive. This new ability to modify how a drive is partitioned without the need to reformat the drive is one of the best features of Disk Utility and is still present in the app to this day.

Adding, Resizing, and Deleting Partitions

Screenshot of Disk Utility partition tool

If you need a slightly larger partition, or you would like to split a drive into multiple partitions, you can do it with Disk Utility, without losing the data that’s currently stored on the drive.

Resizing volumes or adding new partitions with Disk Utility is fairly straightforward, but you need to be aware of the limitations of both options.

In this guide, we’ll look at resizing an existing volume, as well as creating and deleting partitions, in many cases without losing existing data.

Disk Utility and OS X El Capitan

If you're using OS X El Capitan or later, you probably already noticed that Disk Utility underwent a dramatic makeover. Because of the changes, resizing with El Capitan is also a very different process. 

Disk Utility OS X Yosemite and Earlier

If you want to partition and create volumes on a hard drive that doesn’t contain any data, or you’re willing to erase the hard drive during the partitioning process, you can partition the hard drive using Disk Utility.

What You Will Learn

  • The difference between hard drives, partitions, and volumes.
  • How to resize, add, or delete volumes.

What You Need

  • A Mac with OS X 10.5.x or later installed. This guide will work with OS X 10.5 (Leopard) through OS X 10.10 (Yosemite). Earlier versions of Disk Utility are not able to resize or add to an existing volume without first erasing that volume’s contents. Do not attempt to use earlier versions of Disk Utility for this process.
  • One or more drives whose volumes you wish to resize, add to or delete.

Definitions of Partitioning Terms

HDD Open with platters and actuator exposed.
HDD Open with platters and actuator exposed.

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Disk Utility included with OS X Leopard through OS X Yosemite makes it easy to erase, format, partition, and create volumes, and to make RAID sets. Understanding the difference between erasing and formatting, and between partitions and volumes, will help you keep the processes straight.

Definitions

  • Volume - A volume is a storage container that has been formatted with a file system your computer (in this case, a Mac) can recognize. Volumes are logical constructs; they’re not the same as partitions or physical hard drives. Volumes are most often made up of a single hard drive partition that contains a Mac file system. But it’s also possible for a volume to be made up of multiple partitions.
  • Partition - The term ‘partition’ is both a verb and a noun. When you partition a hard drive, you physically create separate sections on the drive; each of these sections is called a partition. A partition defines a specific area of a hard drive.
  • Erase - Erasing is the process of removing all data from a specific volume or hard drive. Data can be erased in multiple ways. The default method on the Mac deletes the data table entries for the location of the file but does not actually remove the file itself from the hard drive or volume. The practical effect of this is that your Mac no longer sees the file, and the space it uses is now marked as available free space. You can also specify optional erase options that will completely remove the data.
  • Format - Formatting a hard drive defines how the hard drive’s media will be laid out to store the computer data. Your Mac can use five different types of formats: Mac OS Extended (Journaled); Mac OS Extended; Mac OS Extended (Case-Sensitive, Journaled); Mac OS Extended (Case-Sensitive); and MS-DOS.

Resize an Existing Volume

Screenshot of Disk Utility partition resize handle

Screenshot Coyote Moon, Inc.

Disk Utility allows you to resize existing volumes without losing data, but there are a few limitations. Disk Utility can decrease the size of any volume, but it can only increase the size of a volume if there’s enough free space available between the volume you wish to expand and the next partition on the drive.

This means that having enough free space on a drive is not the only consideration when you wish to resize a partition, it means the free space must be not only physically adjacent but in the proper location on the drive's existing partition map.

For practical purposes, this means that if you want to increase the size of a volume, you may need to delete the partition below that volume. You will lose all data on the partition you delete (so be sure to back up everything on it first), but you can expand the selected volume without losing any of its data.

Enlarge a Volume

  1. Launch Disk Utility, located at /Applications/Utilities/.
  2. Current drives and volumes will display in a list pane on the left side of the Disk Utility window. Physical drives are listed with a generic disk icon, followed by the drive’s size, make, and model. Volumes are listed below their associated physical drive.
  3. Select the drive associated with the volume you wish to expand.
  4. Click the Partition tab.
  5. Select the volume listed immediately below the volume you wish to expand.
  6. Click the - (minus or delete) sign located below the Volume Scheme list.
  7. Disk Utility will display a confirmation sheet listing the volume you are about to remove. Make sure that this is the correct volume before taking the next step.
  8. Click the Remove button.
  9. Select the volume you wish to expand.
  10. Grab the right-hand bottom corner of the volume and drag to expand it. If you prefer, you can enter a value in the Size field.
  11. Click the Apply button.
  12. Disk Utility will display a confirmation sheet listing the volume you are about to resize.
  13. Click the Partition button.

Disk Utility will resize the selected partition without losing any of the data on the volume.

Add a New Volume

Screenshot of Disk Utility Add New Volume
Screen shot courtesy of Coyote Moon, Inc.

Disk Utility allows you to add a new volume to an existing partition without losing any data. There are, of course, some rules that Disk Utility uses when adding a new volume to an existing partition, but overall, the process is simple and works well.

When adding a new volume, Disk Utility will attempt to split the selected partition in half, leaving all of the existing data on the original volume, but reducing the size of the volume by 50%. If the amount of existing data takes up more than 50% of the existing volume’s space, Disk Utility will resize the existing volume to accommodate all of its current data, and then create a new volume in the remaining space.

While it is possible to do, it is not a good idea to create an extremely small partition. There is no hard and fast rule for a minimum partition size. Just think of how the partition will appear within Disk Utility. In some cases, the partition can be so small that the adjustment dividers are difficult, or nearly impossible to manipulate.

Add a New Volume

  1. Launch Disk Utility, located at /Applications/Utilities/.
  2. Current drives and volumes will display in a list pane on the left side of the Disk Utility window. Since we are interested in re-partitioning a drive, you will need to select the physical drive listed with a generic disk icon, followed by the drive’s size, make, and model. Volumes are listed below their associated hard drive.
  3. Select the drive associated with the volume ​you wish to expand.
  4. Click the Partition tab.
  5. Select the existing volume that you wish to split into two volumes.
  6. Click the + (plus or add) button.
  7. Drag the divider between the two resulting volumes to change their sizes, or select a volume and enter a number (in GB) in the Size field.
  8. Disk Utility will dynamically display the resulting Volume Scheme, and show how the volumes will be configured once you apply the changes.
  9. To reject the changes, click the Revert button.
  10. To accept the changes and re-partition the drive, click the Apply button.
  11. Disk Utility will display a confirmation sheet that lists how the volumes will be changed.
  12. Click the Partition button.

Delete Existing Volumes

Screenshot of Disk Utility Deleting Existing Volume

Screen shot Coyote Moon, Inc.

In addition to adding volumes, Disk Utility can also delete existing volumes. When you delete an existing volume, its associated data will be lost, but space the volume occupied will be freed up. You can use this new free space to increase the size of the next volume up.

The upshot of deleting a volume in order to make room to expand another is that their location in the partition map is important. As an example, if a drive is partitioned into two volumes named vol1 and vol2, you can delete vol2 and resize vol1 to take over the available space without vol1's data being lost. The opposite, however, is not true. Deleting vol1 will not allow vol2 to be expanded to fill the space vol1 use to occupy.

Remove an Existing Volume

  1. Launch Disk Utility, located at /Applications/Utilities/.
  2. Current drives and volumes will display in a list pane on the left side of the Disk Utility window. Drives are listed with a generic disk icon, followed by the drive’s size, make, and model. Volumes are listed below their associated drive.
  3. Select the drive associated with the volume you wish to expand.
  4. Click the Partition tab.
  5. Select the existing volume you wish to delete.
  6. Click the - (minus or delete) button.
  7. Disk Utility will display a confirmation sheet listing how the volumes will be changed.
  8. Click the Remove button.

Disk Utility will make the changes to the hard drive. Once the volume is removed, you can expand the volume immediately above it by simply dragging its resize corner.

Disk Utility - Use Your Modified Volumes

Screenshot of Disk Utility added to OS X Yosemite Dock

Screenshot Coyote Moon, Inc.

Disk Utility uses the partitioning information you supply to create volumes your Mac can access and use. When the partitioning process is complete, your new volumes should be mounted on the desktop, ready to use.

Before you close Disk Utility, you may want to take a moment to add it to the Dock, to make it easier to access the next time you want to use it.

Keep Disk Utility in the Dock

  1. Right-click the Disk Utility icon in the Dock. It looks like a hard drive with a stethoscope on top.
  2. Select Keep in Dock from the pop-up menu.

When you quit Disk Utility, its icon will remain in the Dock, for easy access in the future.