OS X: About Disk Utility's erase free space feature

Disk Utility in OS X includes the ability to securely erase the free space on a hard drive, to reduce the chances of deleted files being recoverable.

There are multiple erasure options that offer different levels of security:

"Zero Out Data" option

This is the quickest free space erasure option, and provides good security. It writes zeros over the unused disk space (one pass). This option takes the least amount of time.
 

"7-Pass Erase" option

Writes data over the free disk space seven times. This provides a highly secure erasure of disk data. A 7-Pass Erase takes seven times longer than a Zero Out Data erase.

Advanced: The 7-Pass Erase option conforms to the DoD 5220.22-M specification. This specification calls for three passes, but Disk Utility performs seven. Click here for more details about the specification.

7 pass overwrite data
(DoD 5220.22-M specification)

Pass

Data written

In binary notation In hexadecimal notation
1 11110110 0xF6
2 00000000 0x00
3 11111111 0xFF
4 Random Random
5 00000000 0x00
6 11111111 0xFF
7 Random Random

Learn more

 If you are erasing the free space of your Mac OS X startup volume, you may see this message:

"Your startup disk is almost full.
You need to make more space available on your startup disk by deleting files."

This is a normal, temporary part of the erase free space process.  Once the free space has been erased, the disk space is available again.
 

Note: With an SSD drive, Secure Erase and Erasing Free Space are not available in Disk Utility. These options are not needed for an SSD drive because a standard erase makes it difficult to recover data from an SSD. For more security, consider turning on FileVault encryption when you start using your SSD drive.

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