In a few weeks Apple will release the next version of OS X - Mountain Lion. As it was with Lion, this will be a download from the Mac App Store. You might think that the only option is to upgrade the current version of OS X that is running on your Mac, but that is not true. You could install Mountain Lion on a separate partition and when the Mac starts up you could choose whether to run your old OS X or the new Mountain Lion. Here's how to install OS X in a separate partition.
When OS X Mountain Lion has finished downloading it will automatically run and begin the installation. At this point it hasn't yet made any changes to the current version of OS X and you can quit the installer. You can run it any time you like by clicking 'Install OS X Mountain Lion.app' in the Applications folder. By quitting the installer after downloading, you can make a backup using Time Machine or a utility like Carbon Copy Cloner.
To install Mountain Lion in a separate disk partition, you need to create one. Run Disk Utility in the Applications/Utilities folder. Select the disk drive in the left pane and the Partition tab in the main part of the window. Unless you have done this before, it will show one partition which occupies the whole disk drive.
Click the plus button below the partition layout diagram and add one new partition. Now there are two boxes, which represent two partitions on the disk (partitions look like separate disk drives to the computer). Click and drag the divider between the two partitions to adjust their sizes. In this example I have a 500Gb disk drive, so I've allocated 200Mb to the first partition labelled Macintosh HD and 300Gb to the second partition labelled Macintosh HD 2. All that is needed is to click Apply and the disk is divided into two partitions. This does not affect any documents or apps or remove any files. It just makes the disk smaller.
Now you can run 'Install OS X Mountain Lion.app' in the Applications folder. It will default to installing on the current disk, upgrading whatever version of OS X is currently running, but underneath the disk drive icon is Show All Disks. Click it and it will show the current disk and the new disk partition you just created. Select this disk partition and continue the installation of Mountain Lion.
There are now two copies of OS X on your Mac, each in separate partitions. There are two ways to select the one you want to use. Start your Mac and after a second or two, hold down the Option key. A list of bootable disk drives is displayed and you can select the one you want to start from. Alternatively, if you want to switch on your Mac and go and get a coffee while you wait for it to start, go to System Preferences and click Startup Disk. A list of disks is displayed and the one you want to start from next time can be selected. This method does not require you to press Option or select the disk to boot from.
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