Working with Disks - Teach Yourself Visually Complete Mac Pro - page 758

Click Erase .
Disk Utility asks you to confirm.
Click Erase .
Disk Utility erases the hard drive.
TIP
What specifically does each of the three secure erase options do to the
data on the hard drive?
The first secure option is a zero-out operation, which means that Disk Utility overwrites the
entire drive once with a series of 0s.
The second secure option is a DOE (Department of Energy)-compliant 3-pass secure
erase , which means that Disk Utility overwrites the entire drive twice with random data and
then a third time with a known data pattern.
The third secure option is a DOD (Department of Defense) 5220-22 M secure erase , which
means that Disk Utility overwrites the entire drive seven times with random data.