How To Install OSX Lion On An External Drive To Test It Out [Mac]

James Bruce 15-07-2011

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 macOS on a PC (Mac Required) With the right combination of components and effort, you can install macOS on a regular PC. 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 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 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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  1. Mark
    October 11, 2011 at 8:46 pm


    I saw this post and was curious if you could answer my questions. 

    My wife and I share an iMac (Intel core 2 Duo). I use a cloned copy of a macbook that I sold 8 months ago. I use the dual boot system (via usb) for my hard drive and my wife uses the internal. I have a ton of music/movies that would take up more space than the internal drive could handle. 

    Could I use your instructions above to upgrade my external HD to Lion? My wife could then continue to use Snow Leopard until she decides to upgrade. 

    Also, does the installer know to automatically restart the system from the external Lion HD or do I have to use the alt/option key when starting for the first time?


    • James Bruce
      October 12, 2011 at 8:24 am

      Hi Mark. That *should* work for your case, but as with doing any reinstalls I would strongly suggest you do a full backup (bootable) before attempting it. 

      I believe it'll install automatically without needing to set the correct drive, but from then on you will need to hold down ALT or choose start up drive from within preferences. 

      Can't stress backing up enough though - I did this witha  fresh install, not an upgrade, and though I dont expect anything would go wrong, it would be far better to be on the safe side. 

  2. LearningCurve
    September 26, 2011 at 9:09 pm

    I wanted to check out Lion on my MacBookPro so I downloaded it, created an install DVD using the InstallESD.dmg file, reformatted a fairly new USB WD PassPort 500GB drive and it took 3 hours before it went through the install process.  Finally it said it was going to reboot to finish installing Lion.  It rebooted and now after 5 hours I am still at an Apple with the Circling Dominoes going round and round and round.  The External hard drive is flashing every minute or so as if it is being accessed but I can't figure this out.  I've done this exactly as noted, not once, but 5 times.  It's nuts.  There is no way it should take this long to install ANY operating system.  I even left it at this exact same state for a maximum of 12 hours and LION still wasn't installed.  Any idea what the deal is?  I formatted it correctly.  Partitioned it as stated and double and then triple checked everything.  After 10 years with Macs I know them pretty well but this has me stumped.  It can't be something from an old system because it's a new, clean install.  The usb drive is new.  I even cleared the PRAM before starting the process.
    Any help?  Anybody?  My sanity is at stake. 

    • James Bruce
      September 27, 2011 at 8:05 am

      I'm afraid you've got me stumped there. I assume you don't have another computer to try again on? I would have suggested putting the installer on a USB stick rather than a hard disk, but it might be too late. If you don't get any other help here, I'd suggest asking in muo answers to crowd source the issue; the apple support forums may be of some use too. 

  3. CY
    August 7, 2011 at 1:01 pm

    If my macbook airs hdd is fried. Can i use the external hdd as the primary and run the lion os off that? or do i have to buy a new hdd for it? also i know u werent talking about hackintosh but if i can sucessfully install the os on the external drive, do u think its possible to boot it on my pc? 

    • James Bruce
      August 8, 2011 at 2:54 pm

      1. Yes, you can boot air off the external drive, though replacing the internal drive would be best. 

      2. No. But you can burn a lion and then try to install it on your hackintosh, outlined here:

    • Claus Frandsen
      October 6, 2011 at 6:18 am

      The Air-problem should be solved, as you mentioned, but the PC-booting-thing will not happen any time soon :)

  4. Grc
    July 26, 2011 at 1:35 am

    The method here didn't work for me.  I have a new USB drive.  Went through all the motions in the article and it ignores the external drive.  It boots into the old internal drive every time.

    If I select the lion drive in Preferences, it ignores it and starts up from the internal drive.

    If I hold down Alt/option on startup, the Lion drive is not in the list.


    • James Bruce
      July 26, 2011 at 11:52 am

      That's very curious. it's definitely listed in the preferences screen? Do you have bootcamp installed?

    • Adiel
      September 6, 2011 at 6:09 pm

      I have the same problem.

      • Tina
        September 14, 2011 at 7:28 pm


        do you have Bootcamp installed?

        • Adiel
          September 14, 2011 at 7:31 pm

          No. I finally got it to work when I reformatted the drive. 

        • Tina
          September 14, 2011 at 7:42 pm

          Ok, great! :)

  5. LuckyLindy
    July 22, 2011 at 1:29 pm

    I want to do something a bit different. I want to see how my main system will end up when I upgrade to Lion without actually doing the upgrade. So I have a SuperDuper clone of my main system on an external Firewire 800 drive. I started to perform the upgrade but saw that it reboots at the end, which I don't want to do because I am busy working on my main machine. Can I defer the shut down and reboot of my main machine until later or does the install immediately shut down my main machine after the upgrade to my external drive? Thanks.

    • James Bruce
      July 26, 2011 at 12:07 pm

      I dont think you can defer restart no. I shut everything down and just ran it before sleeping when I upgraded to final Lion.  

  6. Arne Viman
    July 20, 2011 at 10:13 pm

    One issue I came up against was that from the moment you have begun the USB drive install your ordinary internal drive of your mac will no longer be bootable. Only the Lion install can be booted into. Restarting would not help. This will be in effect until you eventually succeed with the USB drive install.
    Should it fail for some reason and you cannot figure out a way forward then your main drive might remain unbootable until some expert helps you out. Simply trying the ordinary select boot drive option failed, it showed a blank list of no drives to select from.
    Once my USB drive install succeeded the install image got automatically removed from the machine and I was again able to boot into the internal drive when the USB drive remained unattached.
    If someone knows this issue and how to re-enable the internal drive as bootable in case USB install would fail then please comment. In my case my USB install eventually succeeded upon a few retries, great relief.

    • Srwilco
      July 21, 2011 at 6:50 pm

      Can you move the Lion install package to the new drive before you install, or does it have to remain on the drive you are booting from? Would that make a difference?

    • James Bruce
      July 26, 2011 at 12:06 pm

      Interesting problem Arne. Making sure you have bootable cloned drive before trying anything would allow you to fix this easily, I think.

  7. Sascha Kratzer
    July 20, 2011 at 3:19 pm

    I am on it, thx for the help man!

  8. YasuY
    July 16, 2011 at 8:43 pm

    Your way is the exactly same as I would like to do when ion released.I already set external HDD in the way you mentioned here.Thank you alot.

  9. Anomaly
    July 15, 2011 at 8:21 pm

    Just thought I would mention that Linux is also very easy to to do what you just did with the Mac. I carry Linux on a USB thumdrive with me all the time. 

    Another great thing about the portability of Mac OSX is how easy it is to move it from one machine to another. Clone your current set up to a external HDD. Boot that External HDD from another Mac and clone it on to the Mac. You just moved your install to another Mac. I have done this several times.