How to Erase a Macbook's Hard Driveby Joey Perez ; Updated September 28, 2017
Macintosh computers use several methods to erase data on a hard drive. Reinstalling OS X onto a hard drive will clean up a system and erase all data stored on the device. However, not all of the data is completely unusable. Mac OS X will keep the available information stored on the hard drive and write over the data as it obtains more data. To completely erase, or "zero out," the hard drive, you must use your disk utility, and in the security options use any number of erasure options, with the "35-pass erase" option being the most secure. This completely wipes the drive clean, making it nearly impossible to retrieve old data.
Backing Up Data and Accessing the Disk Utility
Back up all of the data that you want to keep before erasing a hard drive.
Insert the Mac OS X installation disc into the CD/DVD drive and restart your computer.
Hold down the "C" key while the system boots up. Select "Choose Installer" and choose "Open Disk Utilities."
Select the hard drive that you wish to erase and click the "Erase" tab.
Select the format and click on "Options."
Select "Don't Erase Data." This option will allow you to erase data on your hard drive, yet keep the data potentially recoverable. The operating system will keep the data but will overwrite it as new data is loaded onto the hard disk. This is the least secure of the options, but it is the quickest.
Select "Zero Out Data." Information on computers is written in binary ones and zeroes. The "Zero Out Data" option will write zeroes over all of the existing data. This is a quick way to securely erase data on your hard drive.
Select "7-Pass Erase." This "7-Pass Erase" method is more secure than the "Zero Out Data." It will write the data on the hard drive seven times for a more complete erasure of the disk. This method takes seven times as long as the "Zero Out Data" method but it is much more secure.
Select "35-Pass Erase." This method is the most secure option available. It will write 35 different binary patterns over all of the data on the hard drive. The "35-Pass Erase" option will take 35 times as long as the "Zero Out Data" method.