Mac Erase Free Space


If you’re selling an old Mac, a spare hard drive, or you’re just quite paranoid about your deleted data, you’re either familiar with—or should be familiar with—the Erase Free Space button on the Erase tab in Disk Utility (found in your Applications -> Utilities folder).

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I actually got everything worked out a bit earlier through some assistance by a very helpful person at Macrumors.com. I ended up using a Terminal command to erase the temporary file that had been created by Disk Utility and then restarted my computer, and the free space showed up. The “as space permits” bit is the key here. When less than 20 percent of the space on your Mac’s startup disk is available — or if less than 5 GB of space is available — your Mac will start automatically purging the oldest Time Machine local snapshots to free up space. The Secure Erase Free Space feature in Disk Utility on your Mac added a layer of privacy and security to file deletion. The feature allowed you to overwrite the free space on a drive to prevent. What the “Erase Free Space” feature did (and still does in prior versions of Mac OS X) was overwrite the free space on a drive to prevent file recovery, adding a layer of security and privacy to file removal, much in the way that Secure Empty Trash performed a similar function of overwriting data after removal.

When you click this button, you’re presented with three options for securely erasing the free space on your hard drive: write over the free space with zeros (fast and relatively safe), write over the free space three times (more secure, very slow), or write over the free space seven times (extremely slow).

I use this feature whenever I sell an old machine. First I format the drive and install a fresh copy of macOS, then I use Disk Utility to erase the free space (typically the one-time write-with-zeros option). This gives me a good sense of security, as it would take a team of dedicated professionals, and possibly special hardware, to have some chance of recovering any of my deleted data—though I really only care about a few financial files, and those are kept on an encrypted disk image, so they’re probably safe anyway.

Use Terminal to securely erase a drive

What if you want to do this from Terminal instead? In Terminal, a program named diskutil provides most of the features of macOS’s Disk Utility.

(Please note that, as with many Terminal commands, there’s a chance of Really Bad Things happening if you make a mistake with the following instructions. Proceed with caution, and make sure your backups are current before you try any of the following.)

To find out about diskutil in detail, type man diskutil at the Terminal prompt. Within the man pages, you’ll find the explanation for how to securely erase a disk’s free space using diskutil:

But how do you figure out what to list for device, which is the disk (or partition) that has the free space you’re trying to securely erase? diskutil can provide that information, too. Just use diskutil list to see a list of all drives and partitions. On the far right, you’ll see an IDENTIFIER column; that column contains the identifier that diskutil needs. Here’s an example of the list output on my machine:


There’s just one last bit of information you need to know to erase the free space on a drive from the command line. In Unix, all devices appear as part of the file system tree, and in macOS, they’re all listed in the /dev directory. So if I want to use diskutil to erase the free space on my Apple_HFS Untitled volume on my external drive, using the single-pass method, the final command would look like this:

diskutil secureErase freespace 1 /dev/disk2s1

Warning! It’s critically important that you include the freespace portion of that command. If you don’t, diskutil will happily start securely erasing the entire disk, instead of just the free space! Yes, that’s a Really Bad Thing, especially because it will be securely erased, meaning there’s no chance you’ll be able to recover the data.

Former Macworld Senior Editor Rob Griffiths founded Mac OS X Hints. He’s now master of ceremonies at Many Tricks Software.
Mac secure erase free space terminal

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If you are planning to sell your Mac or giving it to a third person for permanent use, then you must erase the hard drive first. This is how you can prevent your data falling into wide of the mark hands.

In case, you are using a newer Mac with an SSD, then you have to take extra care.

You can easily ensure in several ways that data saved on your traditional hard drive is securely deleted. But, they do not really help if you have an SSD. As your SSD won’t fall into the category of a hard drive.

AboutSecurely Erasing SSD:

If your Mac comes with an SSD, Apple’s Disk Utility won’t let you to securely erase the SSD. Yes, Secure Erase and Erasing Free Space options are not available in Mac Disk Utility.

People say these options are not required for an SSD since a standard erase itself is enough potential to make SSD data recovery hard. And, some says securely erasing an SSD, can cause wear and tear on its memory cells which affect the drive’s reliability and performance.

Leaving all these discussions, if you just want to securely erase free space on your SSD then here are two different ways you can try.

Before you getting started with it, let me tell something that “this is not really needed; as Apple itself made so hard to secure erase an SSD. But, if you are set on it then, give a second thought to your mind and proceed.

How to Securely Erase a Mac SSD?

Method 1: Steps to Securely Erase a Mac SSD from Terminal

Step 1: Open Applications folder and switch to Utilities folder.

Step 2: Run the Terminal application.

Step 3: Type diskutil secureErase freespace VALUE /Volumes/DRIVE and hit Enter key

Note: Type a number between 0 and 4 in place of VALUE. 0 for single-pass run of zeroes, 1 for single-pass run of random numbers, 2 for 7-pass erase, 3 for 35-pass erase, and 4 for 3-pass erase. Replace DRIVE with the name of your SSD. To run a 7-pass erase in “Peters-Macbook”, use diskutil secureErase freespace 2 /Volumes/Peters-Macbook code.

And if, there is a space in the name of your drive, use a backslash instead of each space (for instance to denote Mac Drive -> Mac Drive).

Method 2: Steps to Securely Erase a Mac SSD Using Disk Utility

Step 1: Reboot Mac, hold the OPTION key and select Recovery partition.

Step 2: Next, in OS X Utilities menu click on Disk Utility.

How To Free Up Space On Mac

Step 3: Select the SSD primary partition and choose Erase option.

Step 4: Under Format, opt for Mac OS Extended (Journaled, Encrypted).

Add-On: Have you accidentally formatted a wrong volume or drive on your Mac machine? Then, try Yodot Mac Data Recovery and unformat the Mac drive or volume easily.

Step 5: Hit Erase button, and set encryption password.

Step 6: Wait till your drive gets erased and encrypted. It takes time, depending on type, size, and speed of the drive.

Mac Catalina Erase Free Space

Step 7: Next opt Disk Utility and choose Mac OS Extended (Journaled).

Now, Erase Free Space and Security Options are accessible (they are not grayed out).

Step 8: Click on Security Options and select 35-Pass Erase.

Free Space On A Mac

Step 9: Hit the OK button and wait until completion of the process.

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