Format an External Disk or a Flash Drive for Mac using Disk Utility in OS X El Capitan and macOS Siera.

To reformat the drive in Mac OS X El Capitan or macOS Sierra :

1. Connect the External disk drive or Flash Drive and open Disk Utility.app from /Applications/Utilities/ folder.
2. Select the External drive or Flash Drive on the left that shows the capacity and not the Volume which appears indented below the drive. Even if you select the Volume, it will also format the Disk, because there is no other Volume on this disk as shown in the picture. You must select the drive name as a correct way to format the disk.
3. Click on the Erase option across the top in the main window.
4. You will be presented with a dropped down panel where you need to type a new name for the Volume and select the format type as OS Extended ( Journaled) and partition map scheme as GUID. OS X Extended (Journaled) is the default Mac file system with GUID partition scheme. Though, you can choose between different partition scheme: GUID Partition Map, Master Boot Record, or Apple Partition Map. GUID is the most recent one, while Master Boot Record (MBR) is older. Both also work with Windows PCs while Apple Partition Map works only with Mac.

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