Get ready to boot
This how-to shows you the steps using Disk Utility 13 in OS X 10.10 Yosemite, but the process is the same if you’re using OS X 10.9 Mavericks or 10.8 Mountain Lion. Here are the instructions using OS X 10.11 El Capitan.
Sometimes you want to wipe out all the data that’s on a hard drive or solid-state drive—erase it and start over. The best way to do this is to format the drive, which both erases the drive and prepares it for storing data by mapping out bad sectors, creating address tables for locating the data on the disk, and more.
Similarly, you may have purchased a new drive that was formatted for Windows out of the box. You'll want to reformat that drive for your Mac.
But formatting a drive so that it can be used as your Mac’s startup drive requires a slightly different procedure than formatting it for use as a secondary drive for storing data.
Step 1: Make a connection
To format an external storage device, connect it to one of the ports on your Mac. Turn the drive on, and make sure it appears in the Finder.
If the drive is internal, it should already show up in the Finder. If not (for example, if you installed a new drive that hasn't yet been formatted), look for it in Disk Utility in the next couple steps.
Step 2: Open Disk Utility
You’ll use OS X's Disk Utility app to format the drive. Locate Disk Utility in Applications > Utilities and open it.
Step 3: Select the drive in Disk Utility
Disk Utility’s left pane show the storage devices connected to your Mac. Underneath each device are the drive’s partitions. Select the drive you want to format. (Chances are, you want to format the whole drive, so you should select the drive itself, not any of its partitions.)
Step 4: Check the partition map
Click the Erase tab if it’s not already selected. At the bottom of the window, you’ll see some information about the drive you have selected. Look at the Partition Map Scheme entry. If it says GUID Partition Table, you can format the drive by selecting Mac OS X Extended (Journaled) in the Format pop-up menu, giving the drive a name, and then clicking Erase. (Remember: This erases everything on the drive!) You can now skip directly to Step 8.
If the Partition Map Scheme says Master Boot Record or Apple Partition Map, you need to continue to step 5.
Step 5: Configure the partitions
Click the Partition tab if it’s not already selected. In the Partition screen, click the Partition Layout pop-up menu and select the number of partitions you want. (In this tutorial, we’re configuring the drive with one partition, the most-common scenario.) Under Partition Information, enter a name for the partition—with a single partition, this is simply the name of the drive. In the Format pop-up menu, choose Mac OS Extended (Journaled).
Step 6: Choose the correct partition scheme
Still in the Partition screen, look at the graphical partition display in the middle of the window. Select the partition you just configured, and then click the Options button. In the sheet that appears, select GUID Partition Table and click OK. This step ensures your drive can be used to boot a Mac.
Step 7: Complete the partition process
Give your settings one last look. And remember: When Disk Utility formats the drive, it erases all the data that’s on the drive. If you're sure that's OK, click Apply. You get one last warning to confirm that you really want to format the drive. If so, click Partition.
Step 8: Wait, and done
It takes a few minutes for Disk Utility to do its thing. When it’s done, your newly formatted drive will appear in the Finder, ready for an OS X installation (or for restoring OS X and your data from another drive).