By default, most computers come with one partition and one volume on that partition. Well, in OS X there’s also a recovery partition, but that’s hidden so we’ll pretend like there’s just one. You can create additional volumes, which are useful for a number of different scenarios. The operation of creating partitions usually involves resizing a partition. That can be somewhat dangerous, so make sure to backup your Mac before doing so.
To create an additional partition (and by default an HFS+ filesystem on that partition), first open Disk Utility from /Applications/Utilities.
Note that by default, the boot volume is highlighted. You can’t create a partition inside a volume or partition, so click on the name of the disk above that.
Here, you can choose to run First Aid, Erase, Mount/Unmount, and Info. Most are unavailable when clicked on a disk, so let’s click on Partition. Doing so shows you each partition on the physical disk.
You can click on each partition to see information about the partition. Let’s click on the plus sign (+) to create our new partition.
When prompted, provide a name for the partition. You can choose a different format for the partition, but let’s leave that as the default for now. Then enter a size and click on Apply.
If you’re taking space away from a partition, the old partition will be resized as a smaller partition, provided that there’s enough free space to do so.
Once the process is complete, you should see your new volume mount.
