Notes about erase methods
Number of Passes
One Pass Zeros or One Pass Random
When using One Pass Zeros or One Pass Random, the number of passes is fixed and cannot be changed. When the write head passes through a sector, it will write only zeros or a series of random characters.
For User Defined method, the user can indicate the number of times the write head passes over each sector. Each overwriting pass will be performed with a buffer containing random characters.
US DoD 5220.22-M
The write head passes over each sector three times. The first time with zeros (0x00), second time with 0xFF and the third time with random characters. Final pass is to verify random characters by reading.
The write head passes over each sector seven times.
Russian GOST p50739-95
The write head passes over each sector five times.
The write head passes over each sector 35 times. For details about this, the most secure data clearing standard, you can read the original article at the link below: http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html