In Disk Utility in Mac OS X 10.4 Tiger, there are four Security Options you can use if you wish to securely erase a volume:
Erases directory information so that data is no longer accessible. The data itself is left unchanged on the disk until its disk space is required, at which point it is written over. Data is potentially recoverable until then. This option is the quickest, but least secure.
Writes zeros over all data on the disk. This option provides good data security in a minimum amount of time.
Writes data over the entire disk seven times. This provides a highly secure erasure of disk data. A 7-Pass erase takes 7 times longer than a Zero Out Data erase.
Advanced information: The 7-Pass Erase option conforms to the DoD 5220.22-M specification. This specification calls for three passes, but Disk Utility performs seven. Visit here for more details on the specification.
7 pass overwrite data (DoD 5220.22-M specification) | ||
Pass | Data written | |
| Inbinary notation | Inhexadecimal notation | |
| 1 | 11110110 | 0xF6 |
| 2 | 00000000 | 0x00 |
| 3 | 11111111 | 0xFF |
| 4 | Random | Random |
| 5 | 00000000 | 0x00 |
| 6 | 11111111 | 0xFF |
| 7 | Random | Random |
Writes data over the entire disk 35 times. This option provides highly effective security against the recovery of data. It requires 35 times more time 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 or here. In the chart below, RLL means Run Length Limited, and MFM means Modified Frequency Modulation.
35 pass overwrite data (Gutmann) | |||||
| 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 | |||
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