Downgrading from macOS Catalina to macOS High Sierra
Did you try the following Recovery key combination as in your case Shift-Option-command-R may help
Command - R: Install the latest macOS that was installed on your Mac.
Option - Command - R: Upgrade to the latest macOS that is compatible with your Mac.
Shift - Option - Command - R: Install the macOS that came with your Mac, or the closest version still available.
See if your High Serria is still available from it. I think it should be as you can still get it from App Store.
You need to first make sure that your model of Mac supports running macOS High Sierra. If your Mac came pre-installed with macOS High Sierra of any earlier version, it can run macOS High Sierra.
Assuming it does, you can't run macOS High Sierra installer from within macOS Catalina (or any newer release of macOS). The error message is conveying precisely the same thing to you:
This copy of the "Install macOS" application is too old to be opened on this version of macOS
To downgrade your Mac by installing an older version of macOS, you need to create a bootable macOS installer on a removable media. Instructions for creating a bootable macOS installer can be found here:
- How to create a bootable installer for macOS
Once you have the installer ready, restart your Mac with the removable media plugged in, press and hold the Option key, and select the installer on the boot screen. Once you are booted with the installer, you can either erase your disk and perform a fresh installation of macOS High Sierra, or partition your disk and install macOS High Sierra in parallel to your macOS Catalina install.
TL;DR
If you have a Mac with a T2 Security Chip, it is easy to downgrade to the any OS
- Just go get the installer from the Apple support page
- Reboot into Recovery Mode by pressing Cmd-R
- Open Utilities -> Startup Security Utility
- Select "Medium" or "No" security
- Reboot and run the installer
Details about Startup Security Utility are explained at Apple's support site.
I am new to this site, so please be kind with edits and comments. I’ve also spent a good amount of time working with Apple Support, to make sure that these methods are
- safe
- don't break any policies or warranties
- work properly
I was given a MPB 16" by Apple to figure out what my DJs for my bands need which is a workable new 16 with the proper drivers while running Mojave so we can still use the plug-ins that are on Ableton and ProTools, not to mention the ones that just haven't been updated to work with Catalina.
As digital obsolescence is just a way of life in the digital era, I have come to grips with that I will eventually have to let some go but I have DJ mixers for example like the Rane 64 which will probably never be updated (at least not anytime soon) since it barely got support for Mojave and I will just have to once the dust settles, go to a Catalina only system but in the meantime, I need a very good band aid that will keep my businesses running.
I am doing this for self but I am keenly aware that, more than any time before, the upgrade and the release of the new 16 at the same time (not to mention the Tower) which I had to freeze 4 $26K versions of that I was ready to buy which is why they gave me this to figure out if I can work but until then, I pulled my deposit and didn't buy.
I will tell you I have successfully created a bootable installer, installed it using the host 16 (therefore while installing, in theory it should have downloaded all the correct drivers for the system so things like video switching, proper thermals, etc are happening) and that is what I am doing right now.
With various diagnostic tools and benchmark tests I am making sure that the system is running as it should before I post my method. The above post is correct but it leaves out some important things and I will hit on them in the video and script tutorials that I will have VERY shortly after I make sure everything is correct.
Just doing it willy nilly is a dangerous operation because the system itself from the 15 to the 16 had some pretty drastic changes and getting the right drivers in the right place is important.
I have been told by several engineers that, contrary to the above statement, that you should run the OS from am external drive only and not create a partition (or actually volume as it's called in APFS file format for macOS. The reasons for that are still unclear to me but I don't want to test it on a system that I don't own and so I am only doing what I am told will keep me in policy and for the record, that does not so do it with care.
I am using a USB 3.1 SanDisk Portable Extreme TB disk and the bottlenecking I was expecting is almost nill and maybe actually be that because of the 10Gbps transfer rates and the ability to have USB 3.1 on the 16 which if you don't install correctly, you might not have.
So I have added some context to my mod deleted post that I am about to post a tutorial on how to do it the way that Apple is helping me do.
I plan on running a dual boot OS for at least 6 months, maybe a year depending on how long it takes some of my developers to start supporting Catalina properly. If I was not able to make this work, I would have had to turn to the dark side and use a Windows system because I can't go backwards in speed (especially in the video CG world where speed is now near real-time previews of major depth in C4D with RedShift which the Navi on the Tower would provide much like the Nvidia RTX 2000+ series).
As for the 16, it will more than support my musical needs.