How to install 10.11.6 on an external drive?

Ok so when you open the InstallMacOSX.pkg and you get the Installation window, this is not installing El Capitan, I mentioned that before, that is why the window says it will take only 7MBs (a full install will take up 20-30GBs of space.). As I said before if you follow the prompts it will create the Install OS X El Capitan.app in your Applications folder. Therefore don't go trying to install it on your external drive, click on the disk you are booted on, i.e your macs internal drive.

When you get the Install OS X El Capitan.app and start the installation, then and only then do you select your external drive.

Why are you using Recovery there is no need for that.

In October 2019 the certificates for several Apple OS's expired, and they have not been renewed on the Recovery servers, that is why you are seeing that error message.

If your mac is capable of running El Capitan you can download it from here.

How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Download OS X El Capitan.

This downloads InstallMacOSX.dmg to your Downloads folder.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the .pkg to the Install OS X El Capitan.app which you will find in 

your Applications folder, it should be 6.2GBs in size.

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

From the installer screens you will asked which disk to install to, select your external drive.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to an external drive for safe keeping. It is also possible to create a bootable USB installer disk, using the Install OS X El Capitan.app in the Applications folder, and the createinstallmedia command in the Terminal app. Read the instructions here,

How to create a bootable installer for macOS – Apple Support

So what exactly did you download, if you had followed my instructions you would get the InstallMacOSX.dmg,

is that what you got, it should be 6.2GBs. This is all you need to do a full install of El Capitan version 10.11.6

follow my instructions as in my first post to get InstallMacOSX.pkg and then create Install OS X El Capitan.app.

Have you formatted your external drive correctly.

Open Disk Utility (Applications> Utilities)

In the left hand panel highlight the external drives Disk icon, not the indented

Volume, click on Erase.

Give the Disk a name

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Map

click Erase.

This should take about a minute. Now your disk should be ready

for the installation.

If you have problems please show screenshots of any error messages etc.