Can disk utility wipe entire Mac hard drive securely? I am running OS X El Capitan 10.11.5.
How many partitions your HDD is divided? Do you want to erase the partition associated with the El Capitan or the entire HDD?
Yes, it can though Mac OS X Utilities. But first I will recommend you take a full backup of the Mac hard drive, SSD or partitions since wiping permanently erases the data beyond the scope of recovery.
Restart Mac OS X and press-hold Command + R before Apple Logo screen. In a short time, you will be welcomed to the “MacOSX Utilities”.
Click Disk Utility. Select your Apple Hard Drive
Click Erase << Security Options << Select range of disk wiping
Click OK << Apply to secure wipe your Mac hard drive.
Alternatively, you can use a third party Mac wipe application to create a bootable media with which you can secure wipe startup disk or any other local volume. But as I mentioned above, before you start wiping any hard drive, you must create at least one backup copy. Other than Time Machine, get a bootable clone of your hard drive.