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

Summary

Learn about Disk Utility's erase free space feature.

Products Affected

Mac OS X 10.5, Mac OS X 10.6, OS X Lion

Disk Utility in Mac OS X v10.5 and later has the ability to securely erase the free space on a hard drive, to reduce the chances of deleted files being recoverable.

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

  • Zero Out Data
  • 7-Pass Erase
  • 35-Pass Erase
     

"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.


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


"35-Pass Erase" option

This writes data over the entire disk thirty five times. This option provides highly effective security against the recovery of data. It takes 35 times longer to perform than the Zero Out option.

Advanced information: The 35-Pass erase option uses the Gutmann algorithm, which means 35 patterns are written to the disk to overwrite data. You can find out more about the Gutmann method here and here. In the chart below, RLL means Run Length Limited, and MFM means Modified Frequency Modulation.

Pass Data written Encoding scheme targeted
In binary notation In hexadecimal notation
1 Random Random      
2 Random Random      
3 Random Random      
4 Random Random      
5 01010101 01010101 01010101 55 55 55 (1,7) RLL   MFM
6 10101010 10101010 10101010 AA AA AA (1,7) RLL   MFM
7 10010010 01001001 00100100 92 49 24   (2,7) RLL MFM
8 01001001 00100100 10010010 49 24 92   (2,7) RLL MFM
9 00100100 10010010 01001001 24 92 49   (2,7) RLL MFM
10 00000000 00000000 00000000 00 00 00 (1,7) RLL (2,7) RLL  
11 00010001 00010001 00010001 11 11 11 (1,7) RLL    
12 00100010 00100010 00100010 22 22 22 (1,7) RLL    
13 00110011 00110011 00110011 33 33 33 (1,7) RLL (2,7) RLL  
14 01000100 01000100 01000100 44 44 44 (1,7) RLL    
15 01010101 01010101 01010101 55 55 55 (1,7) RLL   MFM
16 01100110 01100110 01100110 66 66 66 (1,7) RLL (2,7) RLL  
17 01110111 01110111 01110111 77 77 77 (1,7) RLL    
18 10001000 10001000 10001000 88 88 88 (1,7) RLL    
19 10011001 10011001 10011001 99 99 99 (1,7) RLL (2,7) RLL  
20 10101010 10101010 10101010 AA AA AA (1,7) RLL   MFM
21 10111011 10111011 10111011 BB BB BB (1,7) RLL    
22 11001100 11001100 11001100 CC CC CC (1,7) RLL (2,7) RLL  
23 11011101 11011101 11011101 DD DD DD (1,7) RLL    
24 11101110 11101110 11101110 EE EE EE (1,7) RLL    
25 11111111 11111111 11111111 FF FF FF (1,7) RLL (2,7) RLL  
26 10010010 01001001 00100100 92 49 24   (2,7) RLL MFM
27 01001001 00100100 10010010 49 24 92   (2,7) RLL MFM
28 00100100 10010010 01001001 24 92 49   (2,7) RLL MFM
29 01101101 10110110 11011011 6D B6 DB   (2,7) RLL  
30 10110110 11011011 01101101 B6 DB 6D   (2,7) RLL  
31 11011011 01101101 10110110 DB 6D B6   (2,7) RLL  
32 Random Random      
33 Random Random      
34 Random Random      
35 Random Random      

 

Additional Information

 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 OS X Lion and 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 2 encryption when you start using the SSD drive.

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