install from Catalina

If you have the Install Mac OS X El Capitan.app you will have to create a bootable USB installer.

If you do not have El Capitan get it from here,

How to upgrade to OS X El Capitan – Apple Support

It is a revised version with valid certificates, follow the instructions from Section 4 to Section 7

for downloading the new InstallOSX.dmg making the InstallOS.pkg and then creating the needed

Install Mac OS X El Capitan.app. Using the Install Mac OS X El Capitan.app you create the USB installer.

Once the bootable USB has been created you restart your Mac holding down the option/alt key.

When the Startup Manager screen shows select the USB and press Return.

When the USB has started up you can now start the install of El Capitan, make sure to select the external disk for the install.

How to create a bootable installer for macOS – Apple Support

I can't believe that I have not asked you this earlier, what iMac do you have and what OS did it come with.

If it came with an OS newer than El Capitan then it will not run El Capitan if you manage to get it installed

on your external drive.

Right OK I am booted in to my Catalina partition. I downloaded Mac OS X El Capitan from the How To Upgrade

link I provided yesterday. This downloaded InstallOSX.dmg which looks like this,

Double-clicking on that will get you this,

Double-clicking on that will open an Installation window

where you will see this,

Follow the prompts,

Once the installation has finished look in your Applications folder, you will see this,

Now that you have created and located the Install OS X El Capitan.app

you are now ready to create your bootable installer.

Insert a USB 8GBs or over, into your Mac. Open Disk Utility

click View in the menubar and select Show All Devices.

Highlight the USB disk not indented volume, click on Erase.

Name MyVolume, Format Mac OS Extended (Journaled)

Scheme GUID Partition Map. Click Erase.

Once that has done, open Terminal and enter the following text

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

Press Return,

Enter your Password, press Return

Follow the prompts, cross your fingers.

You want to install Mac OS X El Capitan on an external drive, correct.

Your Mac is running Mac OS Catalina, correct.

The only way to do this is to create a bootable USB installer.

Once that has been created boot to the USB

and install El Capitan.

What do you mean follow blindly, you have asked for advice I have given it.

I have made many USB installers and installed to internal drives and external drives.

Ok so let's try again. Any previously modified folders like Expanded and the InstallOSXMod.pkg can be trashed.

Open the InstallOSX.dmg to get the InstallOSX.pkg, drag it to the Desktop.

Open Terminal type in this text at the prompt,

pkgutil --expand (drag the package here make sure there is a space after expand) ~/Desktop/Expanded2

Press return

After a while a folder will be created on your Desktop called Expanded2.

Open the folder and Control-click on the Distribution file

Select Open With> Other and select Text Edit

The Distribution file will open, scroll down to

var platformSupportValues=["Mac-F2268DC8","Mac-50619A408DB004DA","Mac-F42D86A9","Mac-742912EFDBEE19B3","Mac-942B59F58194171B","Mac-35C1E88140C3E6CF","Mac-77EB7D7DAF985301","Mac-F22C89C8","Mac-35C5E08120C7EEAF","Mac-

Slightly different from before we are going to change one of these Board ID's to the Board ID of your iMac rather than just adding it to the list.

On the second row you will see "Mac-77EB7D7DAF985301",

If you insert your cursor between the 1 and the " then backspace, removing all the characters back to "Mac-77

now enter the numbers and letters for your Board ID so it should look like "Mac-77F17D7DA9285301",

The Distribution file should look like this

var platformSupportValues=["Mac-F2268DC8","Mac-50619A408DB004DA","Mac-F42D86A9","Mac-742912EFDBEE19B3","Mac-942B59F58194171B","Mac-35C1E88140C3E6CF","Mac-77F17D7DA9285301","Mac-F22C89C8","Mac-35C5E08120C7EEAF","Mac-F222BEC8","Mac- etc, etc,etc.

Save the changes and close the Distribution file.

Back to Terminal

Enter the text,

pkgutil --flatten (drag Expanded2 into here, make sure there is a space after flatten) ~/Desktop/ModifiedInstallOSX.pkg

Press Return

Wait it takes quite a while.

Hopefully now it will go through the installation phase, and create the Install OS X El Capitan.app.

One last thing, the Board ID I chose to replace with your iMacs, was chosen because it was similar to yours

so I am guessing it is the identifier for another iMac. If by a cruel twist that Board ID belongs to your

MacBook Pro we might have a problem again. In which case you would need to go through the procedure

again and select another Board ID to replace.

I could not find a list of Macs and their Board ID numbers.

Hello again, so now that you have the Install OS X El Capitan.app in your Applications folder

we should be on the road home. Thought you were trying this on your iMac, from the Terminal window

prompt it mentions MacBook Pro.

Let's start with the USB stick, must be at least 8GBs in size and must be plugged in directly to the Mac,

make sure you have erased it before attempting the createinstallmedia method.

Open Disk Utility, click on View in the menubar, select Show All Devices.

In the left column highlight the Disk not the indented Volume,

click on Erase.

Name MyVolume

Format Mac OS Extended (Journaled)

Scheme GUID Partition Map

click Erase.

If you are still having problems use another USB stick. I too have had a

couple of problems with creating bootable installers in Catalina.

Restarting and trying again and restarting and trying again

got it working.

I need to know the board ID of your iMac

In Terminal can you enter the text below (just copy and paste it after the prompt)

ioreg -l | grep board-id

press Return

Ok so can we go through the Installation section once again.

At this point you have modified the Distribution file, and created the modified package.

Double-clicking on the new package will open the Installation screen.

The first screen will tell you that, "This package will run a programme

to determine if the software can be installed". Press continue.

At the next stage press Continue.

The third screen should say This will take 7MBs of space on your computer.

Click install to perform a standard installation of this software

on the disk "........." Apple could have worded this a bit better.

There should also be a button to Change Install Location, press that.

At this point you select your Catalina disk, this is because you are not about to install El Capitan

what is going to happen is that the installer app, is going to create the Install OS X El Capitan.app.

That is why it says it will take 7MBs, if it was going to install an OS it would be about 30GBs.

So do not select your MacBook Pros Disk.

Select your Catalina disk, press Continue, at the next screen press Install

you will be asked to authorise it with your admin password.

It will now begin to create the Install OS X El Capitan.app in your Applications folder.

After several minutes you should get the Installation completed screen.

Press Close.

Open your Applications folder and locate Install Mac OS X El Capitan.app.

It is this you use to make the bootable installer.

Below are some of the Installation screens.

If you are trying to install Mac OS X El Capitan using the Install Mac OS X El Capitan.app then it will be refused

as you cannot install an older OS over a newer OS, the installer is not intelligent enough to know that you want

to install to an external disk.

Why won't your bootable USB installer not boot up, how did you make it.

Make another one, and see if that works.

Right so that is why it is not going to go through the process of creating the El Capitan install app.

I am going to try to give you instructions to modify the Distribution file in the InstallOSX.pkg.

It may be worth trying.

Ok that's no good, I will try and have a solution for you tomorrow, which will involve

altering the Distribution file and using Terminal to do that, and repackaging it.

Or if you have another Mac running El Cap or earlier, or can borrow one

so you can make that installer app, and then the bootable installer.

But right now it is 4a.m. and I am going to bed.