I am currently running Mac OS X 10.6.8 Snow Leopard on my late 2010 Macbook Pro. I am perfectly fine with running Snow Leopard at present, but I feel I may be missing out on features or indeed updates to the Apple Operating System that maybe being provided to later versions of OS X ie Lion or Mountain Lion, that are not available or being provided to Snow Leopard. Someone told me that Snow Leopard is no longer officially supported by Apple anymore (whether this is true or not, I don't honestly know).

Being lets says a Mac novice, I am apprehensive of upgrading from Snow Leopard to Mountain Lion outright on my Macbook Pro, however I have been told there are a number of other ways to utilise both OS Xes on my computer, without having to forgo one OS X (in my case Snow Leopard).

The one method that interests me is installing & running OS X 10.8 Mountain Lion from an appropriate external hard drive.

I would therefore appreciate step by step guidance if possible on how to install & run OS X 10.8 Mountain Lion from an external hard drive onto my Snow Leopard Macbook Pro.

1 Answer 1


Format and Installation

  1. Connect your drive VIA usb and open Disk Manager
  2. Select the dirve from the left hand list - then click on the "Partition" tab on the top
  3. Choose GUID Partition table (needed for bootable disk)
  4. Name the Drive what ever you like - ad press "Apply"
  5. Insert your OSX disk / mount the image and run the installer.
  6. On the installer select "Show all disks" and choose the new external partition
  7. Install

Booting to External Drive

  1. After installation you will boot automatically into the new instance on the External HD

Moving back to HDD instance

  1. Goto System Preference > Startup Disk > Choose Macintosh HD and reboot - you will be back in the HDD instance.

From here on out Hold Option/Alt on boot-up and you will be prompted with a selection screen for which disk you want to boot from

Hope that helps

  • Also for example if I am already booted into Snow Leopard & then want to change to Mountain Lion, can I also simply connect the external hard drive & change to Mountain Lion that way aswell ?
    – Simon
    Commented Feb 1, 2013 at 14:57
  • You can't jump back and forth from instance - you would need a virtual machine to do that. Because you only have one set of hardware, you can only run one OS at a time. So you can plug in the external drive - but you need to reboot. An alternative is installing Virtual Machine on your physical machine and installing OSX there. You can use VirtualBoxlink it is free and works great! Installation and set up is a breeze
    – tgunn
    Commented Feb 1, 2013 at 15:00
  • Thanks for that. In order to boot to ML from External, do I boot whilst in Snow Leopard or before swtching on my Mac ?
    – Simon
    Commented Feb 1, 2013 at 15:04
  • On Boot up - Hold Option and you will be prompted to choose - ML or SL
    – tgunn
    Commented Feb 1, 2013 at 15:06

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