Just as Disk Utility can securely erase the free space on a disk, it can securely erase an entire disk — visible files and all. Of course, you wouldn't do this with disks you're actively using, but you might do it for disks you're about to sell, give away, transfer to another employee, or otherwise get rid of if they contain any personal data.
If you read the procedure earlier for erasing the free space on a disk, most of these steps should look quite familiar:
Open Disk Utility, which is located in/Applications/Utilities.
In the list on the left, select the volume you want to erase (which can't, obviously, be your startup volume) and then click the Erase tab.
Click the Security Options button to show a dialog box with secure-deletion options, as shown in Figure 13.14.
Click the radio button corresponding to the way you want to erase the disk and then click OK. The default option, Don't Erase Data, isn't what you want because it enables files to be recovered. As with erasing free space, the Zero Out Data ...