How to partition an external hard drive

In a previous tutorial, we covered how to format a hard drive using Disk Utility. We will use the same software to partition the formatted capacity. As long as you don't want to create partitions with less capacity than the data already on the drive, partitioning shouldn't have to require erasing any existing data.

Before you partition a drive, you need to protect your data by copying that drive’s data to another storage device. If you are using Time Machine, you can restore your data from a Time Machine backup after you partition the drive. Be sure to run Time Machine before you decide to partition, so your backup is recent.

Once Disk Utility is done partitioning your drive, the partitions will appear on your desktop as individual hard drive icons. For example, if you partition a 2TB hard drive into three partitions, three hard drive icons will appear.

In this example, we will partition an external hard drive (the 1 TB CalDigit drive in the screenshot) into two partitions. Here is the tutorial on how to partition the startup drive while it's being used to boot your Mac.

Once you’ve copied your data, go to /Applications/Utilities and open Disk Utility. If you are using the Mac OS X Installation disc as the startup disk, click on the Utilities menu and select Disk Utility.

The left column lists the Mac’s storage devices. Listed under each drive are the partitions. For example, in the screenshot here, the first drive listed (320.07 Hitachi) is the internal drive of my iMac. Listed underneath that is the one partition that’s on the internal drive called iMac. The second drive listed (1 TB CalDigit) is an external hard drive with a single partition called 1TB External. The last device listed is the internal optical drive.

There is a set of tabs across the top right part of Disk Utility. To partition a drive, the first step is to select the drive you want to partition in the left column and click on the Partition tab.

The drive is selected in the left column, and I clicked on the Partition tab. Disk Utility shows the current partition of the drive. In the next step, we will define the Partition Layout so that two partitions will be created.

Under the Partition Layout header is a pull-down menu. Click on it, and select the number of partitions you want to create.

When making a selection, Disk Utility automatically sets up the partitions to be equal in capacity. In this example, the two partitions for the 1TB drive I want to create are set up to be 500GB each. In the next step, we will learn how to change the capacities for the partitions.

There are a few ways to adjust the size of the partitions. In the graphic the represents the partitions, there’s a  dot between the partitions. Click and drag up or down to adjust the sizes. To see the actual capacity when you adjust the size, click on one of the partitions so that it has a blue 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. In this example, the first partition will be set to 300GB, while the second will be set to 700GB.

Another way to set your partitions is to use the + and — buttons. These buttons are especially handy if you want to divide a specific partition into more partitions. For example, say you want to create three partitions, one 500GB, and the other two 250GB each. Selecting 3 Partitions in the Partition Layout menu creates three partitions of equal size. Instead, you can select 2 Partitions in the menu, select one of the 500GB partitions, and then click on the + button. This will divide the selected partition in two 250GB partitions, to go along with the 500GB partition. Clicking on the — button removes the partition.

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

Click on the Options button. In this section, you need to select a partition scheme. You have three choices.

For each scheme, Apple provides brief explanations that are clear and understandable. In most cases, you’ll select GUID Partition Table. If you are using a PowerPC-based Mac, you must pick Apple Partition Map if you want to use the partition as a startup drive.

Click OK when you make your selection. This will not start the partition process just yet.

To give your partitions names, click on a partition in the layout, then type the name in the Name box under Partition Information.

You also need to set the format for each partition. The default is Mac OS Extended (Journaled), which is what most Mac users will select. You can also choose Mac OS Extended (Case-sensitive, Journaled), MS-DOS (FAT), or ExFat. (Brief explanations of each format can be found in the How to Format a Hard Drive tutorial.) There’s also a Free Space option; if you select this, the partition will be unused and will not be available to the user unless you format it again using Disk Utility.

Look over your settings one last time to make sure it’s what you want. When you’re ready, click the Apply button.

A confirmation window will appear. It will tell you how the current partition will be modified. Give it a look, and click the Partition button to start the process. This will take a few minutes; the bigger the hard drive, the longer it will take.

A progress bar in the lower right of Disk Utility will appear. When Disk Utility is done, it doesn’t display an alert, but your new partitions will appear on your Mac’s desktop. Now you can install Mac OS X, copy your data back to your new partitions, and use your Mac like usual.