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How to Repartition Your Hard Disk in Snow Leopard and Leopard -- without Losing Data

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Since Mac OS X 10.5 Leopard, Disk Utility has been able to resize and add disk partitions on the fly without erasing data. Maria Langer shows how it works.
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I found myself in a bit of a predicament the other day. I’d done a clean installation of Mac OS X 10.6 Snow Leopard on my new 13-inch MacBook Pro. I’m using the computer as a “test mule” for a book I’m writing about some Mac OS software that’s still in beta.

Unfortunately, the beta software isn’t behaving nicely in Snow Leopard. Although it’ll be fixed by the time the software is ready for release, the developer suggested that I work with Leopard until then. This puts me in a spot, since I’ve been trying to write brief articles about using Snow Leopard. I don’t want to uninstall Snow Leopard from my little MacBook Pro.

Although I got Leopard running successfully from an external hard disk on the same computer, more beta software problems ensued. I couldn’t tell if it was a beta problem or a problem with my weird external boot disk. I had to install Leopard on my internal drive to test.

That’s where Disk Utility’s repartitioning features come into play. Since Mac OS X 10.5 Leopard, Disk Utility has been able to resize and add partitions on the fly without erasing data. This was my solution. And since I took some screenshots along the way, I can show you how I did it -- in case you ever need to.

Warning! Warning! Danger Will Robinson!

You should never mess with the partitioning on your hard disk without first backing it up. Partitioning can go wrong; problems can occur. It is possible to lose all of the data on your hard disk. So before you do this, make sure your Time Machine backup or other backup strategy is up-to-date.

Don’t say I didn’t warn you. And don’t come crying to me if you lose data.

Repartitioning the Disk

You repartition a disk with Disk Utility, so let’s start by firing it up. You’ll find it in the Utilities folder inside the Applications folder on your hard disk (see Figure 1).

Figure 1 Launch Disk Utility by double-clicking its application icon.

The main window of Disk Utility appears. It lists the storage devices -- in this case, the internal Hitachi hard disk and optical drive in my 13-inch MacBook Pro (see Figure 2).

Figure 2 The main window of Disk Utility.

In the list on the left side of the window, select the hard disk to repartition and click the Partition button (as in Figure 3). The partition scheme for the disk you selected appears in the body of the window. The shaded area indicates the amount of occupied disk space.

Figure 3 The partition scheme for a selected disk appears in the window when you click the Partition button.

To resize the existing partition, drag the corner of the partition box up to reduce the size of the box. As you drag, a new value appears in the Size box. (Another way to resize a partition is to simply enter a new value in this box.)

Figure 4 Drag the corner of the partition box to resize the partition.

To add a second partition, click the + button at the bottom of the Volume Scheme area. A second partition appears (Figure 5).

Figure 5 Click the + button at the bottom of the Volume Scheme area to add another volume.

You can rename the new volume by clicking it to select it and then entering a new name in the Name box (see Figure 6).

Figure 6 Enter a new name in the Name box to rename the new volume.

Click the Apply button. A dialog like the one in Figure 7 appears. It confirms that you want to add a partition and resize the existing one. It’s important to note that it clearly states, “No partitions will be erased.”

Figure 7 This confirmation dialog appears before Disk Utility changes the Partition Scheme.

Wait while the disk is verified and the changes are made. A progress bar like the one in Figure 8 appears while Disk Utility works.

Figure 8 A progress bar at the bottom of the Disk Utility window appears as the partition changes are made.

When it’s finished, you should see two volumes on the desktop, in your Finder window sidebars, and in the Disk Utility window (Figure 9).

Figure 9 The Disk Utility window shows both partitions under the disk.

Removing a Partition

If you decide you don’t need a partition anymore, you can remove it.

Simply select it in the Volume Scheme list (Figure 9) and click the button under the list. You can then resize the remaining volume to take up the lost space. When you click Apply and confirm the change, the partition you deleted is removed -- along with all the data on it.


Although the average Mac user may never need to change a hard disk’s partition, it’s good to know that it’s possible to do it without losing any of your data. This can save a lot of time and effort when you need to repartition your hard disk.

Maria Langer is a freelance writer and the author of Mac OS X 10.6 Snow Leopard: Visual QuickStart Guide for Peachpit Press. She blogs at and shares other tips for Mac users at

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