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?
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.