I have MacBook Pro from late 2011 era which I recently upgraded to Yosemite from Mountain Lion. But unfortunately I need some application that only worked on Mountain Lion and I cannot restore back to Mountain Lion as I did not have backup. Now is it possible to install Mountain Lion on USB disk from which I also boot from without actually installing on my MacBook AIR Samsung SSD drive ( the only drive on the system) on which Yosemite is installed and running?

  • do you have a copy of the ML installer ? – Ruskes Jan 23 '15 at 3:49
  • I dont have a copy of Mountain Lion. Thats another thing I have to figure out where to get a copy from but it seems I have to buy from Apple store even though the original Macbook Pro came pre-installed with Lion which was then upgraded to Mountain Lion by purchasing upgrade. – rjc Jan 23 '15 at 4:21

You can buy the ML from App store.

Then install it on USB following these instructions from Apple-HT202796.

Then hold the Option key during boot to select the start up disk.

How ever, running from USB disk can be slow.

Alternative->better solution is to create a new partition (size to be determined) and install the ML on it, if you are using the ML frequently.

  • Thanks for answer. Can you tell me what should be the size of USB disk that wud fit ML plus maybe additional 1 GB? – rjc Jan 24 '15 at 1:32
  • normally 8 GB would be enough for ML it self. – Ruskes Jan 24 '15 at 2:11

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