How to Securely Erase Free Space on a Mac Hard DriveIn Feburary 2016 15,

In Feburary 2016 15,
Launch Disk Utility. It's in your Applications folder under the 'Utilities' folder.
Select your Apple Macintosh Computer's hard drive. If you haven't renamed your drive, it should be 'Macintosh HD.'
Click on the 'Erase' tab.
Click the 'Erase Free Space...' button.
Select the level of security you want to use when erasing the free space on your Macintosh Computer's hard drive. 'Zero Out Deleted Files' is the fastest option and is decent security. It will write over the free space with zeros.'7-Pass Erase of Deleted Files' will write over the free space seven times. This will take longer and offers a higher level of security.'35-Pass Erase of Deleted Files' offers the highest security. It will over-write the free space 35 times and will take the longest. Depending on your hard drive's capacity, this could take several hours to overnight.
Click the 'Erase Free Space' button when you've selected your security level.
In Feburary 2016 15,