It took me longer than I'd like to admit to learn how to use the
secureErase command to shred a disk using diskutil in OS X El Capitan.
# This works, using one pass of random data. # Replace `disk3` with whatever you need! diskutil secureErase 1 /dev/disk3
I found the documentation for the command slightly confusing.
diskutil secureErase [freespace] level MountPoint|DiskIdentifier|DeviceNode
I'm not used to a whole parameter being optional. It turns out that you either use the
freespace flag, or you omit it completely, and that's what the
 around it indicate.
So if you would like to wipe a disk entirely, including free space, simply omit
freespace from your command.