How to Secure Erase / Zero Out external hard drive in El Capitan?

Since this showed up in my search, I figured I would post how to do this from the command line. Note that for my disk, the "Security Options" button does not appear on that screen (it is an old 160 GB drive)...

Step 1: Identify the device using diskutil list. In my case, this was a 160 GB drive that previously held Linux. I underlined my disk below.

$ diskutil list

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *500.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage Macintosh HD 499.4 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

/dev/disk1 (internal, virtual):

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD +499.1 GB disk1

Logical Volume on disk0s2

8FC580CC-1577-4B34-8EC3-9741EE1321C8

Unlocked Encrypted

/dev/disk2 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: FDisk_partition_scheme *128.7 GB disk2

1: Apple_HFS SD Card 128.7 GB disk2s1

/dev/disk3 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk3

1: EFI EFI 209.7 MB disk3s1

2: Apple_CoreStorage Time Machine Disk 999.9 GB disk3s2

3: Apple_Boot Boot OS X 134.2 MB disk3s3

/dev/disk4 (external, virtual):

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Time Machine Disk +999.5 GB disk4

Logical Volume on disk3s2

21DD3F59-ECE6-43BC-BE77-F2B003A241F2

Unlocked Encrypted

/dev/disk5 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: FDisk_partition_scheme *160.0 GB disk5

1: Linux 524.3 MB disk5s1

2: Linux_LVM 159.5 GB disk5s2

Step 2: Use the diskutil secureErase command to erase the disk.

$ diskutil secureErase 1 /dev/disk5

started erase on disk5

[ \ 0%................................................. ] 3% 3:29:40

There's none in El Capitan.

For your external HD, do a format.

There are various workarounds available.

1. You could do this via the command line in Terminal.app just like you can do RAID operations to replace that lost functionality as well
2. You can copy the Yosemite version of Disk Utility.app and hack that in to working
3. You could connect the drive to a Yosemite (or earlier) Mac
4. or You could create a bootable USB stick that does nothing but offer a secure erase facility - see this free tool https://www.paragon-software.com/home/dw-mac/

A fifth option is to encrypt the drive then format it as unencrypted.

Disk Utility is not showing me an obvious place to find an option to Secure Erase (zero out) an external hard drive.

I was able to zero an external drive. And since erasing an external drive was your stated goal, why didn't you select an external drive instead of your internal Apple SSD?

And Disk Utility is not going to knowingly erase the boot drive, which it appears you have selected for you screen shot.

And it turns out disk drives and SSDs make it extremely difficult to perform a true guaranteed secure erase, as the rotating devices perform sector replacement where knowledgeable individuals can recover data from it after a secure erase, and SSDs never write to the sector where the data is stored, then always write to a new sector, and must move the original sector to a garbage collection area, where again knowledgeable individuals can recover your data. As a result it is not wise to declare something 'secure' when it isn't.

Finally, writing zeros on an SSD, besides not actually zeroing what you think you are zeroing (as in it can leave a few gigabytes of your original data still accessible), the zeroing also shortens the life of the SSD. SSDs have a limited number of writes per sector before the material physically wears out. The SSD does wear leveling to help avoid this, but zeroing an entire SSD (or worse 7 or 35 pass random patterns), can seriously reduce the life of the SSD.

With SSDs, it is better to operate them full time as FileVault encrypted drives and then as Barney-15E suggests, just do a reformat which will throw away the old encryption key and then sectors will just be a bunch of random bits. No need to write any zeros and shorten the life of the SSD.

RecoveryPartition was just a partition on an external USB disk. The key being it was on an "External" disk as you said you wanted to erase. Just as your names do not mean anything to me, I don't expect my names for things to matter to you.

If you look at my images, there is a "Security Options..." button that gave me the extra passes. Your screen shot does not show that button.

Now I was using a partition, and you are pointing at the entire disk. That might make a difference to Disk Utility whether it shows the "Security Options..." or not. This was done on an El Capitan system.

To note, sector replacement regarding hard drives has nothing to do with erasing the data on them.

When you secure erase an entire rotating hard drive with Disk Utility, it actually does overwrite each sector/block for as many passes as you choose. When you do a Secure Empty Trash, the areas of the drive where the data is stored are overwritten seven times with random data. There is virtually nothing that can recover anything from a seven pass erase. Not even proprietary lab equipment. And for the typical user, even a one pass erase is as good as gone.

Sector replacement is a highly incorrect term that you'll find all over the web. There is no such procedure. A drive has as many sectors on it that it will ever have when it ships from the factory. It can't create more from out of nowhere.

When you have bad blocks/sectors on a drive, the drive's own firmware will attempt to move the data in the corrupt area of the drive to a new location. Whether it succeeds or not, there is a small area of the drive set aside for mapping out bad blocks/sectors. It's no different than any other part of the drive as far as holding data, except neither the OS or you are allowed to touch it. The drive keeps track of all bad sectors/blocks in the user area of the drive so nothing is ever written to them. As the drive develops more bad areas and are mapped out, the fixed amount of space the drive has to keep track of these areas fills up. When it has no more room to write bad block data, the drive must be thrown away.

There is a secure erase in Disk Utility for non SSD hard drives.

1 - select the disk in the sidebar.

2 - click on the Erase button:

3 - click on the Security Options button:

4 - select the level of secure erase in the next window:

Again, that option does not appear if the disk is an SSD.