On a Windows based computer, you would simply plug in your external hard disk and format it by right clicking and choosing Format option from the Windows Explorer. On a Mac, this is achieved via Disk Utility. It should be a standard practice to securely wipe hard disk before you hand it over to someone or if you’ve just decided to clean it up and use it as new. Note: We are not talking about wiping the built-in disk where the Mac OS X is installed, we’re talking about wiping the external disk only.
Open Finder -> Applications -> Utilities -> Disk Utility or Do a Spotlight search for Disk Utilitiy.
Once you’ve found the Disk Utility, open it. From the left pane of Disk Utility window, select the disk your want to wipe listed in the left pane. As soon as it’s selected, you will see the Erase option in the right pane, from here click the Erase tab and then click Security Options.
In the secure erase options, specify the level of security to be considered when erasing the disk, if this disk had no sensitive data then let it be the default else choose the Most Secure option (will take time to erase disk securely). The Most Secure option is designed to comply with DoD standards. The Less Secure option may leave the data recoverable.
Then choose the Erase-option, you will be presented with a confirmation dialog asking if you really want this drive to be erase, go ahead with it and wait for the erase to finish wiping the disk. Make sure, you’re not wiping the partition from the drive, but the drive itself.