How to and Why You Might Want to Partition Your Mac's Hard Drive

The size of operating systems is growing, as is the size of internal storage. Luckily, the latter has grown faster and now it is possible to run two, or more, different versions of OS X, side-by-side, on the same internal hard drive. And there are good reasons for doing this.
In this tutorial, I will show you how to partition and prepare your Mac’s internal hard drive such that you can boot the machine into your choice of a number of versions of OS X.

The Reason for Multiple Partitions

On the face of it, one might wonder what the purpose of running multiple versions of OS X might be? For most users, without more than a passing interest, there is no reason to partition the internal drive.
For those who are interested, and for those who have a more compelling reason to, then partitioning to run multiple versions of OS X might provide advantages.
If, like me, you consider yourself a bit of a tech-head then being able to run multiple versions of OS X, on one Mac, has its advantages. This is especially true if your Mac is your only computer.
For example, by partitioning my Mac’s hard drive, I have been able to install OS X Mavericks as a separate installation. It runs alongside, but completely separately from, OS X Mountain Lion. This means that I can test the features and functionality of OS X Mavericks without compromising my every day working with OS X Mountain Lion.


For many people, virtualisation is the answer and this generally works well. Being a Mac owner, you are spoilt for choice with VMWare Fusion, Parallels or Virtualbox.
The drawbacks of virtualisation is the cost and performance. The advantage of partitioning the hard drive is that the operating system will be more responsive and you’ve not spent any money on a third-party application in order to do it.

Partition the Hard Drive Screencast

This short screencast describes the partitioning process, outlined in this tutorial, in just a couple of minutes.

How to and Why You Might Want to Partition Your Mac's Hard Drive

How to Partition the Hard Drive

Tip: Do not partition your hard drive without, first, backing up your data. I don’t recommend relying solely on Time Machine, so get a spare external hard drive and perform a clone using software such as Carbon Copy Cloner or SuperDuper!
Partitioning the internal hard drive is a very easy procedure, but you must pay attention to ensure that you do not lose any data. I recommend ensuring that you have a current back up of your data before partitioning.

Step 1. Disk Utility

Open the Disk Utility application which you will find in the Utilities subfolder of the Applications folder. Alternatively, launch Spotlight by pressing CMD SPACE and type disk utility followed by the Return key.
Selecting the hard drive to partition
Selecting the hard drive to partition
Now select your Mac’s internal hard drive to reveal more information about it. Across the top you should see the options First Aid, Erase, Partition, RAID and Restore. If you can see all of these options except for Partition, you have selected the wrong disk in the lefthand column.
Resizing the existing partition
Resizing the existing partition

Step 2. Creating the Partition

Click on the Partition button to reveal the Partition Layout and Partition Information for the hard drive.
It is most likely that you only have one (visible) partition on your hard drive. You can resize the partition by clicking and dragging the bottom righthand side corner of the partition rectangle. The size of the partition, as you drag it, will be displayed.
Alternatively, select the partition that you wish to resize and enter the desired size in the Size box.
Click the + button to create a new partition, select it and enter a name for it. I’d recommend that you choose something other than Macintosh HD.
Creating the a new partition
Creating the a new partition
Ensure that the Format box says Mac OS Extended (Journaled) then click the Apply button. It will then take a few minutes to apply the changes.

Install OS X

Download OS X Lion or Mountain Lion, from the Mac App Store, or locate your download for OS Mavericks and launch it in the normal way.
During the set up screens, pay attention to the destination volume for the software installation, ensuring that you select your new partition.
Ensuring OS X is installed on the new partition
Ensuring OS X is installed on the new partition
Complete the rest of the installation process in the normal way.
Tip: It is not normally possible to install a version of OS X that precedes the release date of your Mac. For example, I am unable to install Snow Leopard and Lion on a MacBook Pro that originally shipped with Mountain Lion. It is possible to install Snow Leopard, Lion, Mountain Lion and Mavericks on my iMac that originally shipped with Snow Leopard.

Booting and Choosing

If your Mac is currently turned off, the way to choose which partition to boot into is as simple as holding down the Option key (sometimes marked ALT) immediately following the start-up chime.
After a few seconds, you will be given a choice of hard drives to select.
If you are currently using your Mac and wish to reboot into the other partition, open up System Preferences by clicking on the Apple on the lefthand side of the menu bar. Select Startup Disk then choose which partition you wish to boot into when you restart your Mac.
Selecting the start-up disc from System Preferences
Selecting the start-up disc from System Preferences
Selecting the volume from which to boot next time the Mac is restarted
Selecting the volume from which to boot next time the Mac is restarted


In this tutorial I taught you how to create partitions, on your Mac’s hard drive, for the purpose of running more than one installation of OS X.
This is particularly useful if you wish to run more than one version of OS X without the drawbacks of virtualisation. It is also useful in order to try new versions of OS X before you decide to upgrade from one to the next.


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