So to get started, first you’ll open Disk Utility—it lives within your Applications > Utilities folder.
The new El Capitan version of Disk Utility is way more…well, jolly somehow than previous versions. It’s bright and colorful, inviting even. Which is a bit weird considering it’s where you’ll go to erase, partition, and repair disks.
Nothing says “let’s have a good old time” quite like the name “Disk Utility,” does it?
Depending on the settings you choose, partitioning can often be done without destroying any data that exists on the disk, but as you may be aware, I’m über-cautious. So I wouldn’t partition a drive that has your only copy of your grandmother’s chili recipe on it without backing up, for example. In fact, I’d say you shouldn’t really mess around with Disk Utility at all unless you’ve got a good backup system in place. Aw, heck, let’s just go ahead and say that if you don’t have a backup right now, you shouldn’t be reading this article. You should be running to the store to get a backup drive like your pants are literally on fire.
Ahem. So when you’re all set with your backups and you’ve got the drive you’d like to partition plugged in, select it from the left-hand list in Disk Utility and click the “Partition” button in the toolbar.
See how I’ve clicked on the disk name itself and not one of the indented volumes beneath it? That’s what you’ll do, because if you don’t, the “Partition” button will be greyed out. Anyway, once you click that button, you’ll see the configuration screen.
Here’s where the magic happens. Click the plus button to add a partition, which you can then name and choose the format and size of. (In most cases, the format you’ll want is “OS X Extended [Journaled],” unless you need to use the “Encrypted” option for extra security.) You can do a whole bunch of other stuff too, like selecting an existing partition and clicking the minus button to delete all of its contents and reclaim the space. And as I mentioned in my intro, you could also set one partition on a drive to be “OS X Extended (Journaled)” and add another one that’s “MS-DOS (FAT).” That MS-DOS partition will let you transport files back and forth between a Mac and a PC if you’ve gotta do that.
After you’ve got things configured just the right way, click on the “Apply” button, and your Mac will walk through the process. At the end, you should see a happy little info window like this:
I’ve clicked on the “Show Details” arrow in the above screenshot, which you can do too if you like seeing the technical info. But afterward, you’re all done, and you can take your disk and do whatever you need to with it. Including backing stuff up. Which I, you know, recommend…a lot. Don’t force me to set your pants literally on fire to make you do it.