I made a new friend over the weekend. Her name is Maureen. Maureen asked me a great question… one that only a few have asked me before. Instead of giving her the answer right then, I asked if she could sit tight until Monday. That way, my answer would not only benefit Maureen, but all my readers who visit this site.
Maureen’s question is this: “I have an old Mac that I want to sell so that I can upgrade to a new computer. How do I ensure that all the data on my old Mac is gone? Like, really really gone?” Fantastic question, and I love that you are concerned about protecting your data. I know too many people who have become victims of identify theft, and in some cases, it could have been prevented by asking this very same question.
The first thing we want to do is make sure that we have a good copy (ideally in two or three places) of your data. If not, you run the risk of losing your data forever. How do you make a good copy? Well, if you are taking advantage of cloudBACKUP services, that's one good copy. If you are using Time Machine as a backup mechanism, that's another good copy. Thirdly, I would also suggest making a manual copy of your data to an external drive (by simply dragging and dropping). I know this may seem overkill, but between the three datasets, you will be good.
With your data backed up, it’s time to wipe your old drive clean! When most people choose to format their internal drive, they perform a simple (and quick) format, which doesn’t really erase the data. Instead, it only gives the illusion that the data is gone by wiping out the drive index information. Anyone with the right software can recover that data with ease (I perform recoveries like this all the time).
In order to really wipe your data for good, you want to boot into recovery mode. If you have a system running OS X Yosemite (10.10) or older, this is pretty straight forward. Simply turn your Mac off, then turn it back on. As soon as you press the power button to turn it on, however, you need to press and hold the Command + R buttons simultaneously. Continue holding the buttons until the Apple logo appears.
If you have a system running OS X El Capitan (10.11), you will need to go a slightly different route. You still need to turn your computer off and back on again. Once you have pressed the power button, you will want to press and hold Alt + Command + R simultaneously. This will take you into an Internet recovery mode. You need to do this because in OS X El Capital, Apple removed the ability to perform a secure format from their Disk Utility application. Booting into an Internet recovery mode will take some time and does require an Internet connection to complete.
You may also consider purchasing a USB recovery key for this very reason (as well as others). It can make your life much simpler.
Once the boot process has completed, you will see a "Welcome to OS X” screen. From here, you will choose to open the Disk Utility application. When Disk Utility is open, you will see your drives listed on the left side. Highlight the drive labelled as “Macintosh HD”. Next, you will choose the “Erase” tab. You will want to rename the drive with the same name (“Macintosh HD”). Next, click on “Security Options”. Here, you will have several options ranging from "Fastest" to "Most secure".
If you choose "Fastest", the drive will be formatted in a way that will not erase the data. This is the default. If you are concerned about your data getting into the wrong hands, this is not the option for you. To ensure that your data is really gone, you will want to slide the slider close to "Most Secure". This will format the drive (wipe the index information) as well as overwrite exiting data with a bunch of garbage data. The closer you move toward "Most Secure", the longer the formatting process will take to complete, but it will ensure that your data is not recoverable. The more passes you tell it to run through, the more garbage is written on your disk. After you have made your choice, click "OK" and then "Erase..." to being the process.
Once the format process has completed, it’s time to reload the Operating System. To do this, simply Quit out of Disk Utility and choose the "Reinstall OS X" option. After a few clicks of the mouse, OS X will be reloaded. When it’s compete, you will see the “Welcome to your new Mac” window where you will be guided to get settled into your “new” Mac. Since the intent is to give or sell this to someone else, there’s not need to go through this process. Simply choose to shut down your Mac, and you are ready to deliver it to the new owner!