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.

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