Partitioning a hard drive, or an SSD drive, involves creating multiple volumes from one physical storage medium. The volumes appear separately in the Finder and are treated separately by your Mac. You can format them independently and use them for different purposes.
You may not know it, but your Mac's boot drive, assuming it's a fairly recent model, is already partitioned. Modern Macs ship with a hidden partition, which hosts OS X's Recovery System, a set of first aid tools and an installer that allows to re-install the OS X on your boot disk if you're unable to boot from it.
There are occasions, however, when you'll want to create another partition, and here we'll show you how.
How to partition your Mac's hard drive or SSD: Why partition your Mac?
There are several reasons why you might want to partition your Mac's hard drive. Historically, the most common was to separate the Mac's system files from the volume on which data was stored. Since OS X came along, with its preference for keeping data in your Home folder, that's much less common.
Now it's more likely that you'll partition a drive in order to run multiple operating systems, or multiple versions of the same operating system, from a single disk. That's how Boot Camp works, by partitioning the drive and allowing you to install Windows on the other partition.
You could also partition your hard drive to allow you to use Time Machine to back your boot partition to a different partition on the same disk. To do this, however, the Time Machine partition has to be at least twice the size of the volume you want to back up, leaving you less than half the total disk capacity to work with. In addition, storing a back up on the same disk as the one being backed up carries several risks and should only be done as a convenient method of restoring older versions of data. Your real back up should always be on a separate disk.
Another reason you might want to partition your Mac is to run the new public beta of OS X El Capitan. Read more about that here: How to install the OS X El Capitan Beta
How to partition your Mac's hard drive or SSD: How to partition your Mac
If you plan to use Boot Camp, you should run Boot Camp Assistant and follow the instructions to partition the drive and prepare for an installation of another OS.
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