Open Disk Utility, select the drive (not the Macintosh HD volume), open the erase tab, set the format to Mac OS Extended (Journaled), and press the erase button.

Start up from the recovery partition by holding command-R.

You can erase the drive and reinstall OS X using the recovery system: The erased volume is written seven times to ensure the data can't be restored.You don't need any external drive. Department of Defense (DOD) 5220-22M standard for securely erasing magnetic media.

Most Secure: This method of securely erasing a volume or disk meets the requirements of the U.S.

It writes two passes of random data to the volume or disk and then writes a single pass of a known data pattern to the volume or disk.

Three-Pass: This is a DOE-compliant three-pass secure erase.

Advanced recovery techniques could restore the data, but it would require a great deal of time and effort.

Zero Out the Data: This erase method writes a single path of zeros to all locations on the selected volume or disk.

It doesn't scramble the file data, which means a recovery app may be able to resurrect the erased data.

Fastest: This is the quickest erase method.The drop-down Secure Erase Options menu presents options similar to the ones in earlier versions of the operating system, but it uses a slider for making choices instead of an options list. Options for OS X Lion Through OS X Yosemite Don't expect this erase method to finish any time soon. It writes over the data on a volume 35 times. 35-Pass Erase: This is the highest level of data erase security that Disk Utility supports.Department of Defense's 5220-22M standards for erasing magnetic media. 7-Pass Erase: This method meets the security requirements of the U.S.The length of time this method takes depends on the size of the volume. It erases the volume's data by writing over it with zeros.

Zero Out Data: This method provides a good level of security.It removes the volume's catalog directory but leaves the actual data intact. Don’t Erase Data: This method is quick but not secure.

Options for OS X Snow Leopard and Earlier

Reformat mac hard drive