OS X Yosemite: Securely erasing your deleted files
When you delete files from a hard disk, the storage space that the files occupied is available for use. However, the deleted files can still be recovered using data-recovery software.
If you want to delete files that contain sensitive information, you can use Secure Empty Trash. Files deleted in this way are completely overwritten by meaningless data.
Drag the items into the Trash, then choose Finder > Secure Empty Trash.
To always empty the Trash securely, choose Finder > Preferences, click Advanced, then select “Empty Trash securely.”
Using Secure Empty Trash takes a bit longer than simply emptying the Trash.
If you want to erase a hard disk and be sure no information is left, you can use one of the secure erase options in Disk Utility.