How to partition a startup drive

When you buy a new Mac laptop, iMac, or Mac mini, the internal startup hard drive is set up with one partition—that is, one large section of storage space. You can use Disk Utility to partition the startup drive (create smaller storage sections), even while the startup drive is being used to boot your Mac. Partitions can help organize your data.

But before you do so, it would be wise to back up your data on the startup drive by copying it to another drive. Disk Utility won’t erase the data, but it doesn’t hurt to be prepared if problems occur.

This example will add a partition on the startup drive of a MacBook Pro. When the process is done, the two partitions will appear on the Mac desktop as two hard drive icons. (We also have details on how to partition an external hard drive.)

When you launch Disk Utility (/Applications/Utilities), the startup drive appears in the left column. Select the drive, and then click on the Partition tab.

Under Volume Scheme, the partition layout map will show a single partition. The blue shaded area represents the amount of storage space that’s currently in use. The white space represents the unused storage space.

Notice that the buttons and menus are greyed out, which means you can’t access them. There is one button you can access, however. It’s the + button. The next step will show how to put the + button to use.

Click on the + button. You’ll notice that the map of the single partition has split in two. This means that you have set up Disk Utility to add a partition on the startup drive.

To adjust the size of the partitions, click and drag on the dot between the partitions in the map. To see the actual capacity when you adjust the size, click on one of the partitions so that it has an outline. Then you’ll see the capacity in the Size box.

You can also select a partition and enter the desired capacity in the Size box.

To add another partition (for example, if you want to create three partitions), select a partition and click on the + button. Clicking on the — button removes the selected partition.

Note that in the partition with the blue shaded area that represents data, you can’t make that particular partition smaller than the space the data is occupying.

You also need to set the format for the new partition. Learn more about the format choices. In most cases, you’ll select Mac OS Extended (Journaled).

Click on the Revert button any time you want to start over from the beginning.

In this example, a second partition of 100GB will be created.

Once you’ve set up your partitions, click on Apply. This won’t start the process just yet.

A confirmation box will appear. It will give you a summary of the actions that Disk Utility will perform. If it looks good to you, click on Partition, and the process will begin.

It’s best to leave your Mac alone and let Disk Utility do its thing. Don’t do anything else on your Mac, like check your e-mail, or visit a Website. It’ll take a few minutes to partition the drive.

Disk Utility will display a progress bar in the lower right of its interface. When Disk Utility is done, it doesn’t display an alert, but your new partitions will appear on your Mac’s desktop. You’re done.