It doesn't scramble the file data, which means a recovery app may be able to resurrect the erased data.

Fastest: This is the quickest erase method.The drop-down Secure Erase Options menu presents options similar to the ones in earlier versions of the operating system, but it uses a slider for making choices instead of an options list. Options for OS X Lion Through OS X Yosemite Don't expect this erase method to finish any time soon. It writes over the data on a volume 35 times. 35-Pass Erase: This is the highest level of data erase security that Disk Utility supports.Department of Defense's 5220-22M standards for erasing magnetic media. 7-Pass Erase: This method meets the security requirements of the U.S.The length of time this method takes depends on the size of the volume. It erases the volume's data by writing over it with zeros. Zero Out Data: This method provides a good level of security.It removes the volume's catalog directory but leaves the actual data intact. Don’t Erase Data: This method is quick but not secure.Options for OS X Snow Leopard and Earlier​