Instead, what we’ll be doing is over-writing the data on your hard drive with gibberish, a random string of bits of data.

Technically, it’s not possible to erase or remove data from hard drives. In Disk Utility, select the external hard drive and then follow the last few steps of these instructions to erase it using one of the Security Options described below. Instead, you can simply open Disk Utility which you’ll find in the Utilities folder inside the Applications folder. If you’re securely erasing an external hard drive you do not need to use a Mac OS X install disk. The instructions below include the option to re-install Mac OS X onto the hard drive at the end of the process. I recommend securely erasing a hard drive before you recycle it or give it away. You’ll need either the Mac OS X install disk that came with your Mac or another OS X install disk that’ll boot your Mac, such as a retail copy of the Mac OS X 10.5 or 10.6 install disk. If your Mac has a solid state drive (SSD), aka Flash Storage, and OS X 10.7 or newer then use these instructions.] Requirements If your Mac has a mechanical, spinning hard drive and OS X 10.7 or newer then use these instructions.

If you’re not sure which type of hard drive you have, then you most likely have a mechanical, spinning, hard drive. Click one of the links below to follow the appropriate instructions for the type of hard drive that you have. Instead, you can startup your Mac from its Recovery Partition and then use its copy of Disk Utility to securely erase your hard. [Update Nov 2014: If your Mac has OS X 10.7 or newer installed then Apple didn’t give you an OS X install disc with your Mac. The process is very similar on older versions of Mac OS X, but the screens will look a bit different. These instructions were written for Mac OS X 10.4 to 10.6.

Here are instructions on how to use Disk Utility to securely erase a hard drive connected internally or externally to your Mac.

Mac disk utility secure erase