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How to Install OS X 10.10 Yosemite on an External Drive

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Sometimes it would be great to be able to try different versions of the OS X operating system before upgrading. This is easy if you have a number of Macs, less so if you have just one. In this tutorial I'll show you how to install OS X on an external drive without replacing the particular version of the operating system, on the Mac, itself.

Whilst this tutorial is entitled How to Install OS X Yosemite Onto an External Volume, the same principles apply for installing Lion, Mountain Lion or Mavericks on to an external volume.

Minimum Requirements

You can install OS X to a choice of media, such as USB drive, SD Card or an external hard drive connected via USB 2.0, USB 3.0, FireWire or Thunderbolt.

It is possible, theoretically, to run OS X from any of these different types of media though–practically speaking–it is probably best to choose USB 3.0 or Thunderbolt. This is down to the transfer speeds possible. With USB 2.0 the experience is quite slow.

You should also note that it is not normally possible to install a version of OS X prior to the version that originally shipped with the particular Mac that you are using.  For example, if the Mac originally shipped with OS X 10.8 Mountain Lion, then it is not normally possible to install OS X 10.7 Lion.

Whilst the OS X installers are much smaller, you will need to factor in approximately 10GB for the installed version of OS X. If you are using a USB Flash Drive, or an SD Card, it will need to be 16GB or greater in capacity.

For the best possible experience, I'd recommend using this technique in the following circumstances:

  • Testing a later version of OS X to the one you are running on the Mac
  • Using USB 3.0 or Thunderbolt external drive, or
  • Using USB flash drive (16GB or greater)

I do not particuarly recommend using an SD Card or a USB 2.0 external drive.  They'll work, but they are quite slow.

Preparation

The process for installing OS X on an external volume is quite straight-forward. Essentially, it's just the downloading of the particular OS X installer required, the correctly formatting of an external volume and the installation of OS X.

Correctly Formatting an External Volume

Connect an external drive
  • Connect an external drive to the Mac.  In this example the drive is a 1TB USB 3.0 external hard drive that is preformatted in the Microsoft File Allocation Table (FAT) format.
Partitioning, naming and formatting the external hard drive
  • Select the hard drive in the left hand pane, as shown, and click the Partition tab 
  • Ensure that 1 Partition is selected int he Partition Layout field
  • If you wish, name the drive in the Name field
  • Select the Options… button
Ensuring that the external drive is correctly formatted
  • Select GUID Partition Table to allow the external drive to be used as a startup volume with a Mac.
Confirming the partitioning, and formatting, of the external volume

Check the confirmation dialogue box and, if in agreement with the proposed action, click Partition to format the external volume.

Downloading the OS X Installer

Downloading an OS X installer from the Mac App Store

Open the Mac App Store application and navigate to the Purchases pane. Provided that you have previously purchased them, you should see Lion and/or Mountain Lion, though they may show as Installed.  Note, Yosemite is only available to registered Apple developers at the time or writing, so you'll not see this option in the Mac App Store.

To download the installer, hold down the Option key and click the Purchases tab.  The button to the right of the app should change to Install.

Use Spotlight to find OS X Installers that are already on the Mac

If that does not work, search for the installer on the Mac, as it may already have been downloaded.  Open Spotlight and type Install OS X to find the required installer.

Install OS X to the External Volume

This example shows how to install OS X 10.10 Yosemite, though the procedure remains the same for OS X 10.7 Lion, 10.8 Mountain Lion and 10.9 Mavericks.

Launch the OS X installer

Launch the OS X installer that was downloaded from Mac App Store. The installer is normally located in the Applications folder. 

You may have found it, instead, using Spotlight.

Read the terms and conditions.  You'll need to Agree to them in order to continue with the installation.

Click the button Show All Disks...

Be aware that only the hard drive in the Mac will be shown by default. Click the button that says Show All Disks… to reveal other volumes that are attached to the Mac.

Select the appropriate external volume onto which OS X is to be installed

Select the external volume to which you'd like to install the operating system.  In this example, I am installing OS X 10.10 Yosemite to the external volume Yosemite 1TB from a Mac that is running OS X 10.8 Mountain Lion.

Installing OS X to the external drive volume

A progress bar will indicate the status of the installation of the software to the external volume.

Click the Restart button to restart the Mac, or wait 30 seconds for a restart

The Mac will then count down from 30 seconds before restarting unless you click the Restart button within this timeframe.

The Mac will then restart

The Mac will then restart and finish the installation process on the external hard drive.

Restarting the Mac

When the Mac restarts, it will reboot into the version of OS X that you've installed to the external volume.  In my example, it's OS X 10.10 Yosemite.

Booting From the External Volume

When you boot the Mac, listen out for the startup chime.  Immediately after the chime sounds, press and hold down the Option key (sometimes marked Alt). 

After a short time, you will be presented with a screen similar to the one shown above.  Use the left and right arrow keys to navigate to the external volume, in my example an external 1TB USB 3.0 drive, and press Enter.

Choosing the Volume From Which to Reboot

Navigating to Apple > System Preferences > Startup Disk

If you are currently using your Mac and wish to reboot to a different volume, open up System Preferences by clicking on the Apple on the lefthand side of the menu bar. 

Select the appropriate startup disk into which to boot next time the Mac is turned on

Select Startup Disk then choose which volume from which you wish to boot you restart the Mac.

Conclusion

Provided that you have a Mac that is USB 3.0 or Thunderbolt capable, and you're using USB 3.0 or Thunderbolt external drives, it is perfectly possible to experiment with different versions of OS X without the need to repartition or upgrade the internal hard drive of your Mac.

Whether you are just curious or wish to explore the differences and features of the different versions of OS X, it is perfectly possible to run OS X from an external volume on a Mac.

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