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install osx on external hard driveOne of the great things about Macs is that the OS is bootable on any Mac machine, regardless of which computer it was originally installed on. This means that if your desktop breaks, you can grab the drive and boot it from your laptop instead, without having to worry about data recovery services and lost work time.

Another powerful use for this is that you can install OSX onto an external hard drive, and have a completely portable OS.  Today I’ll be doing just that with OSX Lion in order to test it out, but this applies just as equally to any previous versions of OSX.

What This Isn’t

This has nothing to do with a Hackintosh How to Install Mac OS X on a PC (Without Using a Mac) How to Install Mac OS X on a PC (Without Using a Mac) Read More or booting OSX on Windows machines. This also isn’t about installing OSX from an external drive (if you can’t read DVD’s for instance). Rather this is about installing a working OSX system on an external drive, so that it’s portable and able to be easily moved around and booted from any machine.

The only limitation is that both machines must use the same architecture. This means you cannot install Lion to a USB drive then boot it on your old Power PC – but you could install it from an Intel i5 based machine and boot from an Intel Core2Duo.

You may be thinking this would useful to run a testbed install of OSX to trial new apps or software, but it’s more designed for the portability aspect of being able to boot the OS anywhere, and to test Lion while still keeping everything on your primary machine as it is. OSX Lion will actually allow you to run two additonal copies of the OS inside a virtual machine VirtualBox's Seamless Mode: Combine Two Operating Systems Into One Desktop VirtualBox's Seamless Mode: Combine Two Operating Systems Into One Desktop Read More for testing purposes.

Why Would I Do That?

If you’re a registered developer like myself, you’re often able to download beta versions of the OS ahead of the official release. Or you might have just downloaded it from a torrent. Regardless, installing beta software on your primary machine is pretty silly, so how about making a test install on an old external drive? You can even import all the settings and files from your main machine, and give all your apps a test run to see if they’re still compatible.


Prepare Your Drive

Connect your USB or other external drive and open up disk utility. Click on the corresponding drive on the left hand side, and be damn sure which one it is because we’re going to erase everything on it.

install osx on external hard drive

Click on the partition tab, select 1 partition from the drop-down box. In order for the drive to be bootable, it’ll need to be formatted with GUID Partition Table, so make sure that’s selected by clicking on Options. If Options is greyed out, you haven’t chosen to make the drive into 1 Partition yet, so re-read this paragraph and do it again.

osx internal installing

Name your drive too if you like, and click Apply when you’re ready. It shouldn’t take too long.


Mount your install image, run the install app, or insert the install CD. Accept the conditions without reading them (whatever, right?), and when asked to choose the drive to install to, be absolutely sure you don’t select your main computer drive. Click show all disks and choose the external drive you just formatted. Proceed to install.

osx internal installing


The install procedure will run through and automatically boot into your new OSX Lion, but when you’re ready to come back to the real world, you can choose the drive to boot from using the Start Up Disk option in System Preferences.

osx internal installing

install osx on external hard drive

Alternatively, to choose during start-up, just hold down the Option/Alt Key on your keyboard. This might not work if you use a wireless keyboard as the Bluetooth stack isn’t initiated until a little while into boot time. If you have an Apple Remote, holding down the Menu key during boot up achieves the same effect though.

Did you know you can boot OSX on any machine? Just another of the advantages of Mac 8 Reasons Why I Switched To A Mac 8 Reasons Why I Switched To A Mac I've been using Macs for about 5 years now, and before that I was a 100% Windows and PC guy. What happened exactly? Why did I switch? Without wishing to start a flame war, here... Read More . Windows installs are tied down to the machine they’re installed on rather than being hardware independent, so if your Windows machine breaks, it’s a long and hard data extraction process awaiting you. Here’s hoping Windows 8 might fix that.

On a side note, have you tried Lion yet? What do you think? Personally, I think the new scroll style is particularly off-putting. On the trackpad I can see myself getting used to it, but on the mouse scrollwheel too? That’s just weird. I am finding the new mission control to be very useful though, especially when I tend to have 20-30 apps open at the same time, but wasn’t ever a fan of spaces. Tying them all together really makes sense.

If you have any questions or comments, I’ll try to respond as best I can but for hardware specific technical questions, you could always try our lively technical support community. We’ve also got thousands of Mac articles in the archives for those of you who are new here.

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