Can I downgrade Catalina to El Capitan using a flash drive? [duplicate]

I have noticed that these instructions no longer work as expected when using the current InstallMacOSX.dmg file, which can be downloaded from either the Apple website "How to get old versions of macOS" or the Apple website "How to create a bootable installer for macOS". These instructions may work as expected, if you are using an older saved version of this file. At best, these instructions are just a hack and Apple has every right to change the InstallMacOSX.dmg file to prevent the desired results.

Note: On 1/31/2021, M. Justin reported this answer still works for Yosemite.

I have posted two alternative answers to this question.

• The first alternative answer involves using a virtual machine. This answer is must less of a hack, because Apple explicitly inserted code into the El Capitan installer to bypass the checking of the model when executed in a "VMware Fusion Player" virtual machine. So, Apple evidently intends for users of newer model Intel Macs to use this new answer to retrieve the Install OS X El Capitan application. Still, I would like know from Apple why they thought such a complex procedure is necessary.

• The second alternative answer involves using an older Mac capable of running Snow Leopard (OS X 10.6).

Below is my original answer.

Note: This answer was adapted from Klanomath's answer to a slightly different question.

There is a link on the Apple website "How to get old versions of macOS" that will download a file that can be used to create the Install OS X El Capitan application. However, if the model Mac is too new to allow the installation for El Capitan, then message given below will occur when trying to create the Install OS X El Capitan application.

This version of OS X 10.11 cannot be installed on this computer.

If you are trying to create a USB installation flash drive of use on another Mac computer, then you can still use the downloaded file to create this installer. The instructions are given below.

This method was verified by using the following configurations.

• Catalina on a MacBook Pro 2017 touchbar for use on iMac 2009 24”.
• Mojave on a Mac mini 2018 for use in a VirtualBox virtual machine on a iMac 2013 21.5".
• Catalina on a iMac 2013 21.5" for use in a VirtualBox virtual machine on the same Mac.

The following steps can be used to create a USB installer for El Capitan.

1. Use instructions on the Apple website "How to get old versions of macOS" to download InstallMacOSX.dmg.

2. Use the Disk Utility to erase a 16 GB or larger flash drive. If the Disk Utility window has a View pulldown menu, make sure Show All Devices is selected. Highlight the flash drive, then click on the Erase button. Use the setting shown in the image below.

   

   Next, click on the Erase button. When finished erasing, click on the Done button.

3. Use the Finder applicaition to open InstallMacOSX.dmg.

4. Enter the commands given below. A folder named El Capitan will be created in the /tmp folder to hold temporary files.

   Note: The easiest way to enter the commands is to copy from this document and paste in the Terminal window.

   rm -rf /tmp/El\ Capitan
   pkgutil --expand /Volumes/Install\ OS\ X/InstallMacOSX.pkg /tmp/El\ Capitan
   diskutil eject Install\ OS\ X
   cd /tmp/El\ Capitan
   hdiutil attach InstallMacOSX.pkg/InstallESD.dmg  -noverify -nobrowse -mountpoint /Volumes/esd
   sudo asr restore -source /Volumes/esd/BaseSystem.dmg -target /Volumes/MyVolume -noprompt -noverify -erase
   

    
   After entering the last command given above, you may get a message similar to the one given below.

   asr: Couldn't personalize volume /Volumes/OS X Base System
   

    
   Some versions of macOS will produce this message while other versions do not. I suspect this is an fatal message because an exit status of 1 was returned.

   Inspection has determined that the part of the command that failed is unimportant.

   Therefore, I was able to ignore this error. However, the error can also be avoided by applying the following steps given in the section below titled Alternative to Using the asr Command.

5. To finish creating the bootable USB flash drive El Capitan installer, enter the commands given below.

   diskutil rename OS\ X\ Base\ System Install\ El\ Capitan
   rm /Volumes/Install\ El\ Capitan/System/Installation/Packages
   cp -rp /Volumes/esd/Packages /Volumes/Install\ El\ Capitan/System/Installation
   cp -rp /Volumes/esd/BaseSystem.chunklist /Volumes/Install\ El\ Capitan/
   cp -rp /Volumes/esd/BaseSystem.dmg /Volumes/Install\ El\ Capitan/
   hdiutil detach /Volumes/esd
   sudo bless --folder /Volumes/Install\ El\ Capitan/System/Library/CoreServices --label Install\ El\ Capitan
   cp /Volumes/Install\ El\ Capitan/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/InstallAssistant.icns /Volumes/Install\ El\ Capitan/.VolumeIcon.icns
   cd "$HOME"
   rm -r /tmp/El\ Capitan
   diskutil eject Install\ El\ Capitan
   

6. Remove the flash drive. You are done.

Alternative to Using the asr Command

The command given below may generate a error message and/or hang depending on the version of macOS and perhaps the model Mac you are using.

sudo asr restore -source /Volumes/esd/BaseSystem.dmg -target /Volumes/MyVolume -noprompt -noverify -erase

You can avoid entering the above command by following the steps given below.

1. If the volume on the USB flash drive is no longer labeled MyVolume, then use the Disk Utility to erase the flash drive. Highlight the flash drive, then click on the Erase button. Use the setting shown in the image below.

   

   Next, click on the Erase button. When finished erasing, click on the Done button.

2. Enter the following command to create a sparse flash drive image.

   hdiutil create -type SPARSE -size 10g -layout MBRSPUD -fs HFS+J -volname MyImage -attach /tmp/El\ Capitan/MyImage
   

    

3. Basically, you want to restore to volume MyImage from the hidden file BaseSystem.dmg. The hidden file BaseSystem.dmg is shown in the Finder window image shown below. The drive OS X Install ESD can be found in the hidden volume Volumes in the root folder.

   There are various ways to view hidden files. These ways depend on the version of OS X or macOS you are using. You may have to search the web for a way that works for you.

   

   Highlight the MyImage volume shown in the Disk Utility main window. Click on the Restore button, then the Image button. Select the BaseSystem.dmg file, as shown below. Next, click on the Restore button.

   

   After a successful restore, you should see the image shown below. You can click on the Done button and quit the Disk Utility.

   

   Also, you should get the following popup.

   

4. Enter the following commands to clone to the flash drive.

   Note: The dd command takes a long time to complete. See this answer to track the progress of the dd command. The bytes to be transferred is approximately 10 GB.

   getdsk() { local "x=$(diskutil info "$2" | grep Part\ of\ Whole)"; eval $1'="/dev/r${x##* }"'; }
   getdsk DSK1 OS\ X\ Base\ System
   getdsk DSK2 MyVolume
   diskutil unmountdisk $DSK1
   diskutil unmountdisk $DSK2
   sudo dd if=$DSK1 of=$DSK2 bs=1m
   diskutil eject $DSK1
   diskutil unmountdisk $DSK2
   diskutil mountdisk $DSK2
   

    

   Note: Since the original will be immediately ejected and eventually deleted, I did not bother to use the hfs.util command to change the UUID of the clone.

You can now proceed with the rest of the steps.  

References

How do I make a bootable USB flash drive on a Mac?
What is the best way to generate a Yosemite or Mavericks Install USB flash drive?
Install El Capitan with VirtualBox on OS X

I was stuck in this very same situation for a while now and have found a way to create an install media (installing while we speak) on MacOS Catalina. A bit of Terminal magic is required, but nothing too fancy.

I will create a temporary folder on the Desktop so it is easier to inspect the structure with finder.

Steps to take to get the command running as suggested in https://support.apple.com/en-sg/HT201372

1. Download and mount the image for El Capitan (e.g. double click onto the .dmg file)
2. Open the Terminal app
3. (optional) Change to the Desktop cd ~/Desktop
4. (optional) Create a temporary folder mkdir installCapitan && cd installCapitan
5. Extract the installer here xar -xf /Volumes/Install\ OS\ X/InstallMacOSX.pkg
6. Change into the resulting folder, with cd InstallMacOSX.pkg
7. Now you're greeted with several files, e.g. Payload, Scripts, InstallESD.dmg
8. Unpack Payload with tar xvzf Payload
9. Now you have the Install OS X El Capitan.app Apple talks about in the link above
10. One more step, move the install data to where it should be: mv InstallESD.dmg Install\ OS\ X\ El\ Capitan.app/Contents/SharedSupport/

Now running the createinstallmedia command should work when given the right paths, and this should work:

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

Change your installation volume path accordingly and be careful to pick the right one, otherwise loss of data may occur.

edit: If your installer fails with something about not being able to verify the install media, check this answer out, it helped me: https://apple.stackexchange.com/a/232016/371520

Good luck

The answer is that there appears to be a problem with Catalina creating an installation app for other versions of OSX-certainly with El Capitan from the issue here. Solution was to follow the Apple supplied procedure https://support.apple.com/en-sg/HT201372 but on another version of OSX-Sierra worked in this case.