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?
MacOS – How to install Mountain Lion on USB disk and boot from USB
dual-bootmacos
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Best Answer
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.