Create a Dual-Boot Mac OS X System Disk

Dual-boot systems are a way of configuring the boot drive so that you have the option to start-up your computer (“boot”) into different operating systems. The most common reason to do this is to have both Mac OS X and Windows available on the same system.

NOTE: While dual-boot systems allow you to choose which operating system you will run, you can’t switch between operating systems without restarting your computer.

However, for me, a big benefit of creating a dual-boot system is that I can have two different Mac OS X systems, with two different versions of Final Cut Pro X, installed. Because all of my media and projects are stored on an external drive, dual-booting allows me to select which version of Final Cut Pro X I want to use for an edit.

NOTE: Actually, you can create any number of different boot disks, to run whatever software you want. Though we use the term “dual-boot,” in point of fact you are only limited by the amount of free space on your boot disk.

DETERMINE HOW MUCH SPACE YOU NEED

Before we create a new partition, we need to figure out how big it needs to be. There are three components:

Most Mac operating systems take 20 GB or less to store. So, let’s estimate the OS at 25 GB, to be safe.

Open your boot disk, select the Applications folder and choose File > Get Info. The total storage space required for all your applications is listed in the top right corner. For this system, applications take 15 GB. Again, let’s round this up to 20 GB.

Finally, open the boot disk, twirl down Users and select your Home directory. (It has an icon of a house.)

Again, choose File > Get Info and write down the size. In this example, I’m using 3 GB for all my Home directory files. Again, we’ll round up to, say, 10 GB.

The size partition you need to create is the sum of these three folders: 25 + 20 + 10, or 50 GB. Your numbers will vary, but this is how you calculate the partition size you need.

CREATE A NEW PARTITION

As shipped by Apple, your boot disk has a single partition. (You can think of hard disk partitions as similar to rooms in a house. Right now, your “house” stores all your files in a single room filled with file cabinets.)

Partitioning allows us to create multiple rooms. The only problem is that all these different rooms must fit into the space of the original house. So, partitioning allows you to create multiple rooms, but it doesn’t expand the total storage space available to you.

NOTE: In the past, we would partition drives to organize our files. This is no longer a good idea, because there’s a performance hit in moving between different partitions. While partitioning the boot drive still makes sense, partitioning for data storage does not.

To create a partition, open Utilities > Disk Utility.

Select the text ABOVE the words “Macintosh HD.” This is because “Macintosh HD” is already a partition. You can’t partition a partition, you can only partition the hard disk that contains the partition.

NOTE: The name of your hard disk – which will probably be a gibberish of letters and numbers – will vary. The key is to select the drive that contains the Macintosh HD partition.

Once the hard disk is selected, click the Partition button at the top center of Disk Utility. If Partition doesn’t appear, you’ve selected the wrong thing in the left-hand panel.

To create a new partition, click the Plus key at the bottom left of the Partition Layout.

Click inside the new partition to select it.

In Partition Information, give your new partition a name. In this example, I called my new partition, “New Boot Disk.” (You can name the partition anything you want, using any combination of letters, numbers and spaces.

Leave Format set to “Mac OS Extended (Journaled).”

And change the size to the size we calculated at the beginning of this article. In this example, I’m using 50 GB; however, your actual number may be different.

Double-check all your entries, then click Apply to create the new partition.

You’ll get a warning message, read it, then click Partition.

After a few seconds, a new hard disk appears on your desktop. Macintosh HD is your original partition, and New Boot Disk is your new boot disk.

INSTALLING SOFTWARE

At this point, you need to install the new operating system on New Boot Disk, along with any applications you want to use.

When you boot into the New Boot Disk you will NOT have access to anything stored on your current boot disk. It won’t be erased, just unavailable. This includes email and everything in your Home directory.

When installation is complete, you are ready to reboot into the new partition.

REBOOTING

Restart your system while pressing the Option key.

After a few seconds a screen appears allowing you to choose which hard disk you want to boot into. Select your new boot drive and click OK.

After a few more seconds, your computer is ready to go – and launched into the new partition.

EXTRA CREDIT – REMOVING A PARTITION

Removing a partition will erase all the data that is stored on it; so be SURE!!! you have moved all essential data to another drive. (Removing a partition will not affect any other partition on the same, or any other, hard drive.

Go back to Disk Utility.

Select the Partition you want to remove and click the minus button in the lower left.

A warning message appears asking you to confirm your choice. Read the message, then click OK.

The partition is removed, but the space it occupied is not reclaimed by the original partition.

To get the space back, drag the thumb in the lower right corner all the way down to fill the gap.

Then, click the Apply button to apply your changes.

SUMMARY

Dual-boot systems are not for everyone. But, when you need to alternate between different operating systems, or different system configurations, a dual-boot system can make that easy.

 


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78 Responses to Create a Dual-Boot Mac OS X System Disk

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  1. pdcgridqa says:

    1. I have macOS Sierra…
    2. I have created new partition of 124 GB from Disk Utility.
    3. Selected new partition and clicked on restore
    4. Selected the .dmg image by clicking on Image
    5. Selected Restore

    I a getting the error message “Restore process has failed. Click done to continue”

    Under details section i am seeing:
    1. Validating target….
    2. Validating source….
    3. Couldn’t open target disk for repartioning.
    4. Operation failed..

    • Larry says:

      pdcgridqa:

      Hmmm… good question. At this point, your best bet is to call Apple Support. They can trouble-shoot and fix this issue for you.

      Larry

      • pdcgridqa says:

        Thanks Larry..
        Is there a way where i can install macOS Sierra and macOS Yesomite on the same macNB? If yes can you please help me with the installation procedure.

  2. EJ says:

    Hi, good article as I am old school mac and recently picked up a like-new iMac (2009) as it turned out when they sent it they forgot to put an OS on it and their excellent customer service people asked what system I wanted and they would send a flash drive installer.

    Since my investment is in Powerbook applications I did some research and discovered Snow Leopard with Rosetta installed can run my PB apps, right? So I am having them send Snow Leopard instal discs and a Yosemite flash install.

    See any issues with this sort of dual boot?

    Thanks in advance.

    EJ

  3. Hi , I have followed the instructions – but with no success – I have a Macbook running 10.9.5 with a 250 gig SSD – it has had the OS updated along the way and now my favourite ( and expensive ) R/C Flight simulator wont work properly . Having been in touch with the supplier they said is not supported into this OS version , but it previously ran perfectly on OS 10.6 . So I have tried to partition part of the disk ( seems fine as its on the desktop ) but the install disk I have for 10.6 has a big circle and slash across the install option and wont allow me to install on the new partition . Any ideas on a solution would be great . Thanks , Ross

    • Larry says:

      Ross:

      This may require a call to Apple Support.

      The OS requires hardware support in order to run. I suspect, but don’t know, that your MacBook is too recent to run older OS’es.

      Larry

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