Erase hard drive content from El Capitan

Try it in Restore mode, boot with CMD+R keys held, if that doesn't work...

Open System Preferences>Accounts, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

In the same pref pane highlight your old account, click the little minus icon, then use Disk Utility to Secure Erase Free Space.

Hi there,

https://www.macworld.com/article/195806/erasefreespace.html