How to Resize a Mac Volume With Disk Utility

Use Disk Utility to change a partition size

Disk Utility underwent a change when Apple released OS X El Capitan. The new version of Disk Utility is much more colorful and — some say — easier to use. Others say it lost many of the basic capabilities that old Mac hands took for granted.

You can still use Disk Utility to change partition size without losing any data, but it's not as easy or intuitive to resize volumes and partitions as it was with the older version of Disk Utility

Information in this article applies to Disk Utility in macOS Catalina (10.15) through OS X El Capitan (10.11).

The Rules of Resizing

Understanding how resizing works in Disk Utility will go a long way toward helping you resize a volume without experiencing any loss of information.

  • When enlarging a volume, the volume or partition that is directly after the target volume must be deleted to make room for the enlarged target volume.
  • The last volume on a drive can't be enlarged.
  • The pie chart interface for adjusting volume size is picky. When possible, use the optional Size field to control the size of a drive segment instead of the pie chart's dividers.
  • Only drives formatted using the GUID Partition Map can be resized without losing data.
  • Always back up your drive's data before resizing a volume.

Fusion Drives that have been partitioned can be resized only with the version of Disk Utility that was initially used to create the Fusion Drive or newer. If your Fusion Drive was created with OS X Yosemite, for example, you can resize the drive with Yosemite or El Capitan, but not with any earlier version.

How to Enlarge a Volume Using Disk Utility

You can enlarge a volume as long as it's not the last volume on the drive. You must be willing to delete the volume that is directly in back of the one you want to enlarge, along with its data.

Make sure you have a current backup of all data on the drive you plan to modify. Here's how to enlarge a volume.

  1. Launch Disk Utility, located at Applications > Utilities.

  2. Disk Utility displays a two-pane interface. Select the drive that contains the volume you want to enlarge.

  3. Click the Partition button on Disk Utility's toolbar. If the Partition button isn't highlighted, you may not have selected the base drive, but one of its volumes.

    The drop-down partitioning pane appears, displaying a pie chart of all the volumes contained on the selected drive.

    The first volume on the selected drive displays starting at the 12 o'clock position; other volumes display moving clockwise around the pie chart. In this example, there are two volumes on the selected drive. The first (named Stuff) begins at 12 o'clock and encompasses the pie slice ending at 6 o'clock. The second volume (named More Stuff) starts at 6 o'clock and finishes up back at 12 o'clock.

    Disk Utility Partitioning OS X El Capitan
    Coyote Moon, Inc.

    To enlarge the Stuff partition, you must make room by deleting More Stuff and all its contents.

  4. Select the More Stuff volume by clicking once within its pie slice. The selected pie slice turns blue, and the name of the volume is displayed in the Partition field.

  5. Click the minus button (-) at the bottom of the pie chart to delete the selected volume.

    The partitioning pie chart shows you the expected outcome of your action. You haven't yet committed to the results. In the example, the selected volume named More Stuff will be removed, and all of its space will be reassigned to the volume to the right of the deleted pie slice, which is named Stuff.

  6. If this is what you want to occur, click Apply. Otherwise, click Cancel to prevent the changes from being applied. You can also make additional changes first.

    One possible change is to control the size of the expansion of the Stuff volume. Apple's default is to take all the free space created by deleting the second volume and apply it to the first. If you would rather add a smaller amount, you can do so by selecting the Stuff volume, entering a new size in the Size field, and then pressing Return. This causes the selected volume's size to change and create a new volume made up of any free space that's left over.

    You can also use the pie chart divider to adjust the size of the pie slices, but be careful; if a slice you want to adjust is small, you may not be able to grab the divider. Instead, select the small pie slice and use the Size field.

  7. Click Apply when you have the volumes (slices) the way you want them.

Resizing Without Losing Data in Any Volume

It would be nice if you could resize volumes without having to delete a volume and lose any information you have stored there. With the new Disk Utility, that isn't directly possible, but under the right circumstances, you can resize without losing data, although in a somewhat complicated manner.

For example, you have two volumes on your selected drive, Stuff and More Stuff. Stuff and More Stuff each take up 50% of the drive space, but the data on More Stuff is only using a small part of its volume's space.

You can enlarge Stuff by reducing the size of More Stuff and then adding the now free space to Stuff. Here is how to do that:

Make sure you have a current backup of all the data on both Stuff and More Stuff.

  1. Launch Disk Utility.

  2. From the right sidebar, select the drive that contains both the Stuff and More Stuff volumes.

  3. Click the Partition button.

  4. Select the More Stuff volume from the pie chart.

  5. Disk Utility allows you to reduce the size of a volume as long as the current data stored on it still fits within the new size. In the example, the data on More Stuff is taking up little of the available space, so you can reduce More Stuff by slightly more than 50% of its current space. More Stuff has 100 GB of space, so you can reduce it to 45 GB. Enter 45 GB in the Size field, and then press the Enter or Return key.

  6. The pie chart shows the anticipated results of this change. If you look closely, you'll notice that More Stuff is smaller, but it's still in the second position, behind the Stuff volume. You have to move the data from More Stuff to the newly created, and currently untitled, third volume on the pie chart.

    Before you can move the data around, you have to commit to the current partitioning. Click Apply.

  7. Disk Utility applies the new configuration. Click Done when it's complete.

Moving Data Using Disk Utility

  1. In Disk Utility's sidebar, select the untitled volume you just created.

  2. From the Edit menu, select Restore.

  3. The Restore pane drops down, allowing you to copy the contents of another volume to the currently selected volume.

    In the drop-down menu, select More Stuff and then click Restore.

  4. The restore process takes some time; how much depends on the amount of data that needs to be copied. When the process is complete, click Done.

Finishing the Resizing

  1. In the Disk Utility sidebar, select the drive that contains the volumes you've been working with.

  2. Click the Partition button.

  3. In the partition pie chart, select the pie slice that is immediately after the Stuff volume. This pie slice is the More Stuff volume you used as the source in the previous step. With the slice selected, click the minus button (-) below the pie chart.

    to remove the selected volume and add its space to the Stuff volume.

  4. No data is lost because the More Stuff data was moved (restored) to the remaining volume. You can verify this by selecting the remaining volume and seeing that its name is now More Stuff.

  5. Click Apply to finish the process.

Resizing Wrap-Up

As you can see, resizing with the new version of Disk Utility can be simple, as shown in the first example, or convoluted as in the second example. In the second example, you could also use a third-party cloning app, such as Carbon Copy Cloner, to copy the data between the volumes.

So, while resizing volumes is still possible, it has become a multi-step process. Nevertheless, Disk Utility can still resize volumes for you; just plan ahead and be sure to have current backups.