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. Other methods to clear Purgeable disk storage on Mac. Since Purgeable disk space is storage the Mac has determined can be cleared out when needed, simply using the Mac will typically cause the purgeable storage capacity to change (both growing and shrinking. Manually clearing caches and temporary files can sometimes reduce the purgeable space.

Click here to return to the 'Erase free disk space from the command line' hint

Wouldn't you be much better off using /dev/random instead?

'Wouldn't you be much better off using /dev/random instead?'
No, that would actually be terrible. First of all, you shouldn't fill the boot disk of a running machine because it will cause problems, so the hint is a bad idea anyway. However, if you are anxious to do bad stuff like fill your root drive, then you definitely shouldn't do it from /dev/random, because /dev/random requires vastly more CPU power to generate data for than /dev/zero does. If would take few hours to fill a drive from /dev/zero, but it could take a few weeks to fill it from /dev/random.
Also, I don't know if Mac OS X does this, but Linux will sometimes block on reads from /dev/random while it waits for more genuinely random input from the outside world (say mouse movements, or network traffic rates or some such). (This used to cause 'depleted entropy pool' problems for ssl and ssh on linux. Sorry for the 'in my day' storry, but I remember when I used to use linux to generate SSH keys and I would have to wiggle my mouse around a lot in order to generate ssh keys in a reasonable amount of time. :-)

Yes, I remember when I setup a FreeBSD server ~5 years ago, I got the standard generating key message you would normally get, then something odd telling me to 'wiggle my mouse or hit keys on the keyboard'. You feel like an idiot standing at a console in the server room, 'wiggling the mouse', and banging on the keyboard, with other admins* walking around asking you if you have gone insane.
Its funny now, not so much back then.
* They were Windows admins, I was the *nix admin at that time.

Firstly one does not run this as root. run this as a normal user. The system reserves some space for the root user, and this way you will not run out of disk space for critical system. (At least I know this to be true for reiser, ext3 and ext2 filesystems on Linux.
Secondly, anyone paranoid enough to want to do it should never use /dev/zero (Not even when doing it multiple times) since you can still get the data using forensic techniques. Even if you do this multiple times... some drives with advances caching may not even do the successive writes on a very low level (even with only a small sized disk-cache).
Lastly, yes, /dev/random is more cpu intensive... as for a lot more? No it does not:
I did:
(cat /dev/zero > zero &) ; (cat /dev/random > random &) ; sleep 20 ; killall cat ; ls -l zero random
And it showed:
joseki:~ marius$ ls -la random zero
-rw-r--r-- 1 marius staff 222302208 Apr 24 12:27 random
-rw-r--r-- 1 marius staff 666898432 Apr 24 12:27 zero
So, yes, I would suggest using random... a good system-wide anti-entropic engine should be pretty efficient.

As UNIX SysAdmin, I don't think this makes any sense at all.
First, filling up your drive is a very bad idea, as Rob points out. Many services will stop functioning or exhibit odd behavior.
Secondly, I'm not quite sure what you're trying to do here? Make sure any unused sectors are zero'd out? I'm guessing you're using secure delete already anyways. This just doesn't make any sense. Do you work for the NSA? Didn't think so. You don't need to 'zero' your unused disk space.
P.S. - This '35 pass delete' stuff is B.S. The only way to recover data from a HD after 3 passes is with a team of people and highly specialized hardware. Sorry everybody, but my guess is that your data is not that important. Save your hard drive life and stop using 35-pass deletes. If you have sensitive data, encrypt it using a strong passphrase and you'll be much better off.
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Nem W. Schlecht
http://geekmuse.net/

Not only is this not a good idea, it doesn't entirely work as advertised. Realize that changes to files are often not written to disk immediately. Instead, they are held in memory (cached) and queued for writing to disk. So, when your cat command dies due to insufficient space, it's likely that some of the writes have not completed. When the file is removed, those writes can be deleted from the queue of pending writes. Consequently, some of the disk blocks will never be overwritten.
Also, many filesystems limit the amount of disk that a regular user (i.e. not root) can write to. For instance, UFS reserves 10% of a filesystem (by default, can be set with tunefs minfree option) for root. This would mean that the above command would fail after writing to 90% of the disk, not 100% (leaving 10% unwritten). Unfortunately, I'm not familiar with the implementation of HFS+, so I can't comment on whether it has similar behavior.

'Do you work for the NSA? Didn't think so. You don't need to 'zero' your unused disk space.'
I'm guessing you know the things I'm about to say already and just didn't think about them when you wrote this, but using secure erase means that Finder is configured to do secure erase when it empties the trash. It has no impact on, say Quicken's temporary files in /tmp . File in /tmp are also outside of the user's FileVault (assuming this person is using FileVault). There have also been multiple cases where people have suffered identity theft after their financial information was lifted from old drives despite the fact that it had already been (insecurely) deleted on the old drive.
With that said, FileVault, secure erase, and encrypted VM will cover the vast majority of people's needs. In fact, simple lack of familiarity with HFS+ on the part of thieves will cover a lot. Nonetheless, there are legitimate cases for a non-FBI-employee to want to erase the blank space on a root drive.

some of us do. given, this isn't the best way to go about it, it would help prevent some data recovery.
if you want to use this method, i would recommend booting into single use mode first. (cmd-opt-s)
diskutil is the correct way to do this.
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Disk Utility Tool
Usage: diskutil secureErase [freespace] level
MountPoint|DiskIdentifier|DeviceNode
Securely erases a disk or its freespace.
Level should be one of the following:
1 - Single-pass randomly erase the disk.
2 - US DoD 7-pass secure erase.
3 - Gutmann algorithm 35-pass secure erase.
Ownership of the affected disk is required.
Example: diskutil secureErase 2 /dev/disk2
Note: Level 2 or level 3 secure erases can take an extremely long time.

er,
diskutil secureErase freespace 1 /dev/xxx

So, if you change the hint to: then the file will magically disappear as soon as the drive fills up. This is slightly safer since it does not fail if RAM fills up to. For example, there are reasons why a rm call might not complete. In a ram-tight situation (if, say, the filesystem cache were to get rather large), then a fork might fail if there's no room on the swap device for more vm. JP

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Pell

As robdew pointed out, you can do this with diskutil. A good rule of thumb is that if Disk Utility can do it, diskutil can do it.
In fact, Apple is great at letting you do anything (and more!) from the command line - for example, the 'softwareupdate' command for auto-updates.

The right method to perform this very action is part of diskutil:
diskutil secureErase freespace 1|2|3 /dev/disk#s#
(See the man page on diskutil)
Band-aids never work right. They always peal off!

But that won't work unless you have root access, right?

This command does exactly what Disk Utility does. If your computer crashes during a wipe of the free space, you will find that your hard disk space is reduced. Removing the zeroed file from /tmp (if I remember correctly) will restore the disk space. Try doing it in Disk Utility and watch your disk space shrink.
As others have pointed out, some disk space is reserved for root, and therefore the chance of a crash is minute.
Thanks, robdew, for pointing out the secureErase option of diskutil. Some how I missed that. It does exactly the same thing.

...if you're concerned about running the aforementioned 'erase free space' operation on a 'live' filesystem, you may want to shut down your system and reboot with your OSX install disc and run Disk Utility from there instead.
Or, (in keeping with the spirit of the original hint of using the cmd-line) reboot with your OSX install disc into Single User mode (documented elsewhere) then use the diskutil tip mentioned earlier.

Erase Free Disk Space Mac Os

The Optimised Storage features of your Mac help you save storage space by storing your content in iCloud and making it available on demand:

• When storage space is needed, files, photos, films, email attachments and other files that you rarely use are stored in iCloud automatically.
• Each file stays exactly where you last saved it, and will be downloaded when you open it.
• Files that you’ve used recently remain on your Mac, along with optimised versions of your photos.

If you haven't yet upgraded to macOS Sierra or later, find out about other ways to free up storage space.

Find out how much storage is available on your Mac

Choose Apple menu  > About This Mac and then click Storage. Each segment of the bar is an estimate of the storage space used by a category of files. Move your pointer over each segment for more detail.

Secure Erase Free Space

Click the Manage button to open the Storage Management window, pictured below.

Manage storage on your Mac

The Storage Management window offers recommendations for optimising your storage. If some recommendations have already been turned on, you will see fewer recommendations.

Store in iCloud

Free Disk Space Downloads

Click the Store in iCloud button and then choose from these options:

• Desktop and Documents. Store all files from these two locations in iCloud Drive. When storage space is needed, only the files you've opened recently will be kept on your Mac, so you can work offline easily. Files stored only in iCloud display a download icon , which you can double-click to download the original file. Learn more about this feature.
  
• Photos. Store all original, full-resolution photos and videos in iCloud Photos. When storage space is needed, only space-saving (optimised) versions of photos are kept on your Mac. To download the original photo or video, just open it.
  
• Messages. Store all messages and attachments in iCloud. When storage space is needed, only the messages and attachments you recently opened are kept on your Mac. Find out more about Messages in iCloud.

Storing files in iCloud uses the storage space in your iCloud storage plan. If you reach or exceed your iCloud storage limit, you can either buy more iCloud storage or make more iCloud storage available. iCloud storage starts at 50 GB for US$0.99 a month, and you can purchase additional storage directly from your Apple device. Find out more about the prices in your region.

Optimise Storage

Click the Optimise button to save space by automatically removing films and TV programmes you've watched. When storage space is needed, films or TV programmes that you've purchased from Apple and have already watched will be removed from your Mac. Click the download icon next to a film or TV programme to download it again.

Your Mac will also save space by only keeping recent email attachments on this Mac when storage space is needed. You can manually download any attachments at any time by opening the email or attachment, or saving the attachment to your Mac.

Optimising storage for films, TV programmes and email attachments doesn't require iCloud storage space.

Empty Bin Automatically

Empty Bin Automatically permanently deletes files that have been in the Bin for more than 30 days.

Reduce Clutter

Reduce Clutter helps you identify large files and files you may no longer need. Click the Review Files button and then choose any of the file categories in the sidebar, such as Applications, Documents, Music Creation or Bin.

You can delete the files in some categories directly from this window. Other categories show the total storage space used by the files in each app. You can then open the app and decide whether to delete files from within it.

Find out how to re-download apps, music, films, TV programmes and books.

Where to find the settings for each feature

The button for each recommendation in the Storage Management window affects one or more settings in other apps. You can also control these settings directly within each app.

• If you're using macOS Catalina or later, choose Apple menu  > System Preferences, click Apple ID and then select iCloud in the sidebar: store in iCloud will turn on the Optimise Mac Storage setting on the right. To turn off iCloud Drive entirely, deselect iCloud Drive.
• If you're using macOS Mojave or earlier, choose Apple menu > System Preferences, click iCloud, then click Options next to iCloud Drive. Store in iCloud turns on the Desktop & Documents Folders and Optimise Mac Storage settings.
• In Photos, choose Photos > Preferences and then click iCloud. Store in iCloud selects iCloud Photos and Optimise Mac Storage.
• In Messages, choose Messages > Preferences, then click iMessage. Store in iCloud selects Enable Messages in iCloud.

• If you're using macOS Catalina or later, open the Apple TV app, choose TV > Preferences from the menu bar, then click Files. Optimise Storage selects “Automatically delete watched films and TV programmes”.
• If you're using macOS Mojave or earlier, open iTunes, choose iTunes > Preferences from the menu bar, then click Advanced. Optimise Storage selects “Automatically delete watched films and TV programmes”.
• In Mail, choose Mail > Preferences from the menu bar and then click Accounts. In the Account Information section on the right, Optimise Storage sets the Download Attachments menu to either Recent or None.

Empty Bin Automatically: From the Finder, choose Finder > Preferences, then click Advanced. Empty Bin Automatically selects “Remove items from the Bin after 30 days”.

Other ways that macOS automatically helps to save space

Erase Free Disk Space Mac

With macOS Sierra or later, your Mac will take these additional steps automatically to save storage space:

How To Free Up Space On Mac

• Detects duplicate downloads in Safari, keeping only the most recent version of the download
• Reminds you to delete used app installers
• Removes old fonts, languages and dictionaries that aren't being used
• Clears caches, logs and other unnecessary data when storage space is needed

How to free up storage space manually

Even without using the Optimised Storage features described in this article, you can take other steps to make more storage space available:

• Music, films and other media can use a lot of storage space. Learn how to delete music, films and TV programmes from your device.
• Delete other files that you no longer need by moving them to the Bin, then emptying the Bin. The Downloads folder is good place to look for files that you may no longer need.
• Move files to an external storage device.
• Compress files.
• Delete unneeded email: In the Mail app, choose Mailbox > Erase Junk Mail. If you no longer need the email in your Deleted Items mailbox, choose Mailbox > Erase Deleted Items.

Learn more

• The Storage pane of About This Mac is the best way to determine the amount of storage space available on your Mac. Disk Utility and other apps may display storage categories, such as Not Mounted, VM, Recovery, Other Volumes, Other, Free or Purgeable. Don't rely on these categories to understand how to free up storage space or how much storage space is available for your data.
• When you duplicate a file on an APFS-formatted volume, that file won't use additional storage space on the volume. Deleting a duplicate file only frees up the space required by any data that you might have added to the duplicate. If you no longer need any copies of the file, you can recover all of the storage space by deleting both the duplicate and the original file.
• If you're using a pro app and Optimise Mac Storage, find out how to make sure your projects are always on your Mac and that you're able to access their files.