Looking for a way to install and run OS X on an external hard drive? This can be useful for several reasons. First, it lets you run another copy of OS X without the need for an additional Mac.
Also, because you can run a full copy of OS X on an external drive, it can be used to troubleshoot other Macs or as a virtual OS X. I’ve already written about how to install OS X in VMware Fusion, but it takes up space on your Mac. By using an external drive, you can save space on your Mac, although this can be a little slower if you’re using USB 2.0.
In this article, I’ll walk you through the requirements and steps to install OS X on an external hard drive.
Format an external hard drive
The first thing you need to do is format your external hard drive properly. The file format must be Mac OS X Journaled and you must use a GUID Partition Map. To do this, open Disk Utility and connect the disk to your Mac.
Under “External” on the left menu, click the external hard drive, and then click the Erase button. Make sure to back up all data before erasing the disc. When you click “Erase”, a dialog box appears in which you can adjust some parameters.
Give your disk a name, select OS X Extended (Journaled) for the format, and a GUID Partition Map for the schema. It only takes a couple of minutes to clean and format the drive. Your drive is now ready for OS X.
Install OS X
You can install OS X to an external hard drive in two ways: by reinstalling OS X from the OS X Utilities Recovery screen, or by downloading OS X from the App Store and running the installer. I’ll show you both methods in case one doesn’t work for you.
The easiest way is to download OS X from the App Store. When you open the App Store, you will see on the right a link to the latest version of OS X (El Capitan at the time of this writing).
Go ahead and click the Download button to start downloading the installer. Please note that if you already have this version of OS X installed, you will see a pop-up message asking if you want to continue or not. Just click “Continue”.
Once downloaded, simply double-click the installer, which will be located in the Applications folder.
Keep clicking past the license agreement, etc., until you reach a screen that asks which drive to install OS X on. By default, it’s a MacBook.
Click the Show All Drives button and you will see an icon for various drives and partitions on your Mac. I named my external hard drive OS X and it appears in the middle.
You can also tell it is an external hard drive by the orange hard drive icon. Click “Continue” and follow the instructions to complete the installation. Please note that your computer may reboot during installation and you do not need to do anything. OS X will automatically proceed with the installation to the external hard drive instead of booting to your internal version of OS X.
At the end of this article, I’ll show you how to boot from an external hard drive, so skip it if you ended up using the App Store method. Note that by default the Mac will boot directly from the external hard drive until you change it.
The second way to install OS X is to restart your Mac, press and hold the COMMAND + R keys. This will load OS X Recovery.
The OS X Utilities screen appears and here you want to click Reinstall OS X. You will go through a few main screens again, but when you get to the hard drive screen, click Show All Disks again.
Using this method, you will need to sign in with your Apple ID and password so that the entire OS X installer can be downloaded from Apple servers. Whichever method you choose, it will take 15 to 30+ minutes to install OS X on an external hard drive.
During OS X installation, your computer will reboot several times. Note that when it finally boots into OS X, this is the version running from your external drive. To switch between the internal and external drives, you must restart your computer and hold down the OPTION key.
When you do this, you should see at least four icons. In my case, there are five of them because I have Windows installed using Boot Camp. Anyway, the gray MacBook and Recovery 10.11.2 icons refer to my internal OS X, and the orange OS X and Recovery 10.11.3 icons refer to the version installed on my external drive.
Use the arrow keys to select the drive to boot and then just press Enter. If you have a newer Mac and a USB 3.0 capable USB drive, this should work pretty quickly. Overall, this is a fairly straightforward process and it took me less than an hour to get everything working. If you have any questions, do not hesitate to comment. Enjoy!