Note that if you already have that version of OS X installed, you’ll see a popup message appear asking if you still want to continue or not. Go ahead and click the Download button to start downloading the installer. Once you open the App Store, you’ll see a link on the right for the latest version of OS X (El Capitan as of this writing). The easiest way is to download OS X from the App Store. I’ll show you both methods in case one isn’t working for you. There are two ways you can install OS X on to your external hard disk: by reinstalling OS X from the OS X Utilities repair screen or by downloading OS X from the App Store and running the installer. It should only take a minute or two for the drive to be erased and reformatted.

Give your drive a name, choose OS X Extended (Journaled) for Format and GUID Partition Map for Scheme. When you click Erase, a dialog will pop up where you can configure some options. Make sure you backup any data before you erase the drive. Under External in the left hand menu, click on your external hard drive and then click on the Erase button. To do this, open Disk Utility and connect the drive to your Mac. The file format has to be Mac OS X Journaled and you have to use the GUID partition map. The first thing you’re going to need to do is format the external hard drive properly.

Mac terminal format from windows disk