Updated 2023-06-11 !
The use case is that I had an old iMac, max OS is High Sierra, that crashed
and it didn't work with an Internet or Time Machine restore.
The other MAC:s I have are running Catalina or Big Sur and if you try to
download and older OS, via app-store, you just get an error saying that the running OS don't accept the
download of an old OS.
The universal way to create an USB installer
seems to be using the command "createinstallmedia", requires OS X 10.7 Lion or later, but you need the respective OS install app !
A new, better ??, without Python dependences is found here.
Getting the Installer app
There is a fantastic Python program which will download files, from Apples servers, and create a menu of available MacOS. There you can select the one you want, which will download additional files, and create a "Install_macOS_xxx.dmg" file.
Open a "Terminal" window and issue the command
curl https://raw.githubusercontent.com/munki/macadmin-scripts/master/installinstallmacos.py -o installinstallmacos.py
or an alternative
curl https://raw.githubusercontent.com/grahampugh/macadmin-scripts/main/installinstallmacos.py -o installinstallmacos.py
to get the program and then execute it with
sudo python installinstallmacos.py
which gave the following list(31 October 2021)
# ProductID Version Build Post Date Title
1 001-15219 10.15.5 19F2200 2020-06-15 macOS Catalina
2 001-04366 10.15.4 19E2269 2020-05-04 macOS Catalina
3 071-97382 11.6 20G165 2021-09-17 macOS Big Sur
4 001-36801 10.15.6 19G2021 2020-08-12 macOS Catalina
5 002-23774 12.0.1 21A559 2021-10-25 macOS Monterey
6 002-23589 11.6.1 20G224 2021-10-25 macOS Big Sur
7 061-86291 10.15.3 19D2064 2020-03-23 macOS Catalina
8 041-91758 10.13.6 17G66 2019-10-19 macOS High Sierra
9 041-88800 10.14.4 18E2034 2019-10-23 macOS Mojave
10 001-68446 10.15.7 19H15 2020-11-11 macOS Catalina
11 061-26589 10.14.6 18G103 2019-10-14 macOS Mojave
12 001-51042 10.15.7 19H2 2020-09-24 macOS Catalina
13 071-72781 11.5.1 20G80 2021-07-26 macOS Big Sur
14 001-57224 10.15.7 19H4 2020-10-27 macOS Catalina
15 041-90855 10.13.5 17F66a 2019-10-23 Install macOS High Sierra Beta
16 061-26578 10.14.5 18F2059 2019-10-14 macOS Mojave
17 071-78704 11.5.2 20G95 2021-08-18 macOS Big Sur
18 001-36735 10.15.6 19G2006 2020-08-06 macOS Catalina
In this case I selected "8" and the application started to download the files. At the end you will get a message that the "Install_macOS_xxx.dmg" file is created. In a "Finder" window open the "Install_macOS_xxx.dmg" file and it will mount and create a folder with the "xxx.app" file.
Formatting the USB
Get an USB with at least 16 Gb storage and format it via "disk
utility.app". Press "Cmd + 2" to show all devices and not only the default partitions. Format with name "macOS", "Mac OS
Extended(journaled)" and "GUID Partition Map" as the Scheme. Inspiration link.
Create the USB installer
Open a "Terminal" window and start type "sudo " and then draw the "xxx.app" file, from the finder window mentioned above, to the terminal
window which will give the path. Add "/Contents/Resources/createinstallmedia --volume /Volumes/macOS" to the command line. This will give you
something like
sudo /Volumes/OS\ X\ Base\ System/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/macOS
Hit "Enter", and fill in the admin password. It will take some time and at the command line you will see something like
Ready to start.
To continue we need to erase the volume at /Volumes/MacOS.
If you wish to continue type (Y) then press return: Y
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
Copy complete.
Making disk bootable...
Copying to disk: 0%... 10%... 20%... 30%... 40%... 50%... 60%... 70%... 80%... 90%... 100%
Install media now available at "/Volumes/Install macOS XX"Done.
You can check the write progress in the "activity Monitor.app". "Disk" tab, process "createinstallmedia" and written bytes.
Install the new OS
After the completion I ejected the USB, inserted it to the iMac and turned on the power. During boot the "Option" key was pressed and held down, and then I selected the USB
media.
I first formated the disk, clean install, with "Mac OS Extended(journaled)", since it was an old non SSD HD, and
then back to the installation. (With en SSD use APFS/GUID)
Please note !!! that if you have a Mac with he T2 security chip you can’t boot, by default, from en external or removable media without changing security.
After the installation don't forget to update the OS to get the latest
security patches.
Remarks
Info
Additional info about the "installinstallmacos.py" is found here.
Delete image
After the image is built please note that the folder "content" can be deleted. This folder is used to temporary store the download. If the download is interrupted it don't have to start from the beginning.
No Python ?
App download alternative
An alternative is to use the work by dosedudes1. I have used his Mojave patcher to force an update on a MacBook Air, 2011, which has Mojave as max OS.
To get the High Sierra install app, to the iMac, I downloaded the "High Sierra Patcher", on one of my working Mac:s, and then in the Meny used the "Tools => Download macOS High Sierra" to get the install app.
Then follow the steps above.
Monterey on unsupported Mac
If you are aiming for Monterey here is a post where I "forced" an upgrade to an old, 2011, MacBook Air.
Inspiration link
On this link you also will get additional methods and links to older Mac OS
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