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.