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. Thank you for using Apple Support Communities.The first thing you’re going to need to do is format the external hard drive properly.

After the partition is erased, choose Quit Disk Utility from the Disk Utility menu.

A dialog box appears asking if you want to erase the partition: click Erase.

If you have installed Microsoft Windows using Boot Camp, or have multiple partitions, this procedure will erase the partition you select. Important: This step will completely erase your hard drive.

Select a volume to erase (usually called Macintosh HD).

Click the Utilities menu, the choose Disk Utility.

Select a language, then press the right arrow key.

If your are using a wireless mouse, a mouse icon should appear asking you to turn the mouse on so that the computer can discover it.

Alternatively, press and hold the Option key at startup to select the volume containing the installer. The computer will start up using the Install media.

Press and hold the 'C' key while the computer starts up.

Insert the Mac OS X Install DVD into the optical drive, or insert the MacBook Air Software Reinstall Drive into the USB port.

Important: This procedure will completely erase your hard drive.

Mac OS X v10.6 Snow Leopard: How to Erase and Install If you have completed the applicable steps in the article you came from, the next step is going to be to erase the hard drive to prepare it for re installation of the Operating System: It sounds like you are wanting to clear your personal info from your computer before you sell it.

Imac using os x boot disk to wipe drive