How to Securely Erase Drives on Mac

If you’re looking to get rid of or reuse an external drive, you can go to Disk Utility to format it.  However, did you know that you’re not actually erasing anything?  While you may not be able to see any files, that drive can still have those files on it, and someone with the know-how and a few apps may be able to recover the files.  If you want to make sure that the external drive, you’ll want to wipe it in a secure fashion.  We’ll show you how.

Not Secure By Design?

You may be asking yourself why don’t the files get erased when I tell it to wipe the drive, and this is understandable.  Back when drives were much smaller, they did in fact do this.  However, as we began to work with drives that were starting to use Gigabytes of storage, it began to take longer and longer to erase these drive.  In the name of speed, the quick erase function was developed.  This is also true on other systems like Windows and Linux.  When you wipe a drive this way, your Mac (or other computer) simply tells the drive to just ignore whatever is on there and let it be written over.

It’s kind of like painting over a wall with some art or graffiti on it.  If you paint over it you won’t see what’s underneath, but the original paint is still there.  Someone with special tools or who knew how to chip paint at a wall may be able to recover parts of it.  Again, this analogy isn’t perfect, but it makes the point.  Unless you want to be waiting hours, maybe even days for a drive to erase, you’re going to want to use quick erase.

Another reason is with the rise of solid state drives, which includes things like flash drives, erasing caused wear on the drives.  Unlike the older spinning drives, a solid state drive can only be written to so many times.  Every time you save a file, add something, etc. that part of the drive where the file is store will wear out a little bit and eventually won’t be usable.  Fortunately most good flash drives and other solid state storage mediums wear out after millions of uses, but each little bit helps.

In a lot of cases, this quick erase is fine.  You probably wouldn’t care if someone recovered an old school essay from your drive.  But certain personal photos, financial documents, or other files with sensitive info are things you probably don’t want to be recovered.

How to perform a secure erase

Go ahead and plug your external drive into your Mac.  Then open the Disk Utility app by opening the Applications folder, then the Utilities folder, and finding Disk Utility in there.

Once opened, should see the drive connected to your Mac in the left sidebar of the window.  The drives toward the top should be under the “Internal” header, while your external drives will be under the “External” header.

A picture of Disk Utility with the flash drive to be erased selected.
The flash drive we want to erase is selected in Disk Utility under the “External” tag

Select the flash drive you want to erase.  Note you can also do this for a single partition of the external drive by selecting that partition instead.  In this case, I want to erase the whole 4 gig drive.  Now we’ll hit the “Erase” button in the header of the Disk Utility window.

Screen Shot 2020-03-15 at 4.22.44 PM
The Erase window, with the “Security Options” box in the bottom left corner

A new window should pop up.  Here, you can give you drive a new name in the “Name:” box, as well as select the format you want to make the drive use.  If you’re using this only on Macs, select the Mac OS Extended (Journaled) or APFS options.  If you’re going to be using this on another machine, you’ll want to select the ExFAT or FAT options instead for inter-compatibility.  If you hit Erase now, it will only perform the quick erase.  Instead, hit the Security Options button in the bottom left corner of the screen.

Screen Shot 2020-03-15 at 4.23.44 PM
The Secure Erase window and its 4 options in the slider

A new window labeled “Security Options” should now be on screen with a slider.  The slider has 4 positions, with Fastest being on the left, and Most Secure on the right.  The Fastest option if the aforementioned quick erase. Moving the slider over one will actually erase over the drive twice: once with random data, and once with all 0’s.

The next option is one compliant with the standards set by the United States Department of Energy.  It erases the drive 3 times with the first erase being random, and the other two erasures being with some predetermined set of data, though not necessarily 0’s.  This is generally a good enough erasure for most people with things like their personal financial documents and the like, though consult your local laws.

The last and most secure erasure follows the standards set forth by the United States Department of Defense 5220-22 M Standard.  It wipes over the drive 7 times with Random Data.  This is the best one to use in cases business financials, medical records, etc.  However again this is not legal advice, and you should always consult your local laws.  That said, if it follows the US DOD guidelines, it’s probably plenty for the most secure files you have or the ones you don’t want to recoverable.

Select whatever option is most appropriate for you on the slider, and then hit the OK button.  You’ll be taken back to the Erase window, and from here you can hit the Erase button to begin erasing your drive. The erase window will disappear and will be replaced by a progress bar showing you how far along it is in the process.  You can also hit the “Show Details” button to see specifically where the process is in the erasing and what option was selected.

A window with a progress bar and detailed description of the erasing of the flash drive.
Erase progress window

The more secure the option you picked, and the larger the drive you’re erasing, the more time it will take.  For the 4 GB flash drive I was using on the 2-erase option, it took about 15 or so minutes to complete.  Your time will vary accordingly. Congratulations, you’ve now securely erased your external drive.

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