Can NOT downgrade on iMac with Preinstalled Catalina

rtcentinc

macrumors newbie
Original poster
Jan 5, 2017
3
0
Unless I’m missing something.. :-/ ..you can not downgrade from a Mac (in my case, 27″ iMac) that came with Catalina preinstalled.. at least not like in previous OS versions.. it will not allow the reboot/dualboot.. I’ve tried, with bootable Mojave, High Sierra, El Capitan and even Snow leopard drives.. even was with Apple on the phone doing it… we wiped both HDs clean, changed the formats, etc, etc, etc,… the only way, is to go deep in the firmware, wiping it, which will void your warranty..
running Cat 10.15.3, 2TB fusion drive, i9 processor, radeon 48
 

cmaier

Suspended
Jul 25, 2007
18,029
16,011
California
Unless I’m missing something.. :-/ ..you can not downgrade from a Mac (in my case, 27″ iMac) that came with Catalina preinstalled.. at least not like in previous OS versions.. it will not allow the reboot/dualboot.. I’ve tried, with bootable Mojave, High Sierra, El Capitan and even Snow leopard drives.. even was with Apple on the phone doing it… we wiped both HDs clean, changed the formats, etc, etc, etc,… the only way, is to go deep in the firmware, wiping it, which will void your warranty..
running Cat 10.15.3, 2TB fusion drive, i9 processor, radeon 48
Yes, I believe that is correct.
 

vertical smile

macrumors 601
Sep 23, 2014
4,612
5,707
I always thought you could install the OS that came with the model at its launch, regardless of what OS is installed at the time of purchase.

They originally had 10.14 at its launch, I would think that you should be able to install Mojave.

I’ve tried, with bootable Mojave, High Sierra, El Capitan and even Snow leopard drives
Can you elaborate with this statement? What kind of drive? How did you try them?

even was with Apple on the phone
Apple phone support is hit or miss, I often know much more about Apple products than the support people I have chatted with in the past.
- - Post merged: - -

I search online and quickly found someone that was in your situation and found a solution. It looks like it can be done relatively easily without messing with firmware, but does require another Mac.

I think you can find an alternative to using DiskMaker X using Terminal, Apple used to have instructions on how to make an installer using Terminal on their support page.

It is in the comments section of this:


You can give it a try:

Some 2019 iMac owner said:
Here is the solution. If you have an iMac like mine and it was built in 2019 and it came with Catalina installed, you CANNOT get Mojave to install from Recovery. Recovery will only install the current OS or if you go to internet recovery it will reinstall the OS that was originally installed on your Mac. If it was Catalina to begin with Catalina is what you will get. There is a solution though if you need to or want to get rid of Catalina and install Mojave. 1) Find someone who has a Mac running Mojave. Then launch Safari and go to https://support.apple.com/en-us/HT210190. Next get a copy of DiskMaker_X_803…not the Catalina versionit is free and can be downloaded with Safari. Install it on the donor Mac. Use this app to create a bootable Flash-Drive. Next take the Flash-Drive to your Mac that has Catalina on it. Plugin the Flash-Drive. Restart with the Option Key held down. Choose the Flash-Drive as the Startup Drive. Once the Mac has restarted Open Disk Utilities on the Flash-Drive Mojave Installer. Delete all of the Containers on your drive. Next create a new Container…name it. Quit the Disk Utility. Next choose Install new OS. Mojave will appear. Click install. Soon you will have Mojave on your Mac. Catalina will be gone!!!No need for special Terminal Commands etc.
 
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Fishrrman

macrumors Core
Feb 20, 2009
19,442
6,736
OP:

You didn't tell us WHICH iMac you have.
2017?
2019?

I suggest you try the following.
You will not know if it works UNTIL YOU TRY IT, so...try it.

Backup any personal data on the iMac in question first.

Then, do this:
Reboot to INTERNET RECOVERY using the following command:
shift-OPTION-command-R

This will boot to internet recovery and make available for installation the earliest version of the OS that originally shipped with the computer (which I believe will be Low Sierra for a 2017 iMac and Mojave for the 2019 iMac).

Next...
DO NOT run the OS installer yet.
Instead, open Disk Utility.
IMPORTANT IMPORTANT IMPORTANT:
Go to the view menu and select "show all devices"
Now, select the topmost item in the list that represents the physical drive inside the iMac.
Then, erase it.
If you are going to install Low Sierra, erase to Mac OS extended with journaling enabled, GUID partition format
If you are going to install Mojave, erase to APFS, GUID partition format.

Once the erase is complete, open the OS installer and install the OS.

That should get you to Low Sierra (on the 2017) or Mojave (on the 2019).
If you want Mojave on the 2017, you can now download and Mojave installer and update that way.
 
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Riwam

macrumors 65816
Jan 7, 2014
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Just for your information.
Although not what Apple advises and linked to the need of using a clone of your main volume, a clone which must be APFS formatted to install Apple security updates, it is NOT IMPOSSIBLE to install Mojave in your main drive and keep HFS+ if wanted. I placed a thread on how it went in my case as well as the difficulties implied.
As already said, NOT any kind of advise and NOT at all saying you should do it, but just an additional info that it is possible... if for any reasons wanted.
 

StevePaselli

macrumors newbie
Jun 6, 2016
18
4
Italy
Maybe this will help: disconnect your Mac from network (wifi, ethernet, whatever) and set the system clock to a date after the release date of whatever macOS you’re installing (but before the next major macOS release) before booting it from a usb key with the installer. This way you can always install the version of macOS that was current at the time that Mac model was released.
To install Mojave on a brand new iMac 27 you only need to obtain the macOS installer and use the createinstallmedia command to put Mojave on the usb key. Just remember to reset the date afterwards, and after reconnecting make sure to install every macOS update available for Mojave.
 

fisherking

macrumors 604
Jul 16, 2010
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Of course. I doubt the OP has not thought of this.

I am not sure what the OP's issue with Catalina is, but there is a lot of software that will stop working once upgraded to 10.15.
sure, but that's how it works. at some point, the OP may want to (or need to) update their OS. so am just suggesting they start where they are, and move forward. anyway, just a thought....
 

bogdanw

macrumors 65816
Mar 10, 2009
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Having once upon a time installed OSX on a Pentium 4 PC, I genuinely find hard to understand how macOS can not be installed on an iMac.
Is it the T2 chip? That can be disabled.
Is it the Firmware folder or Firmware.scap file in the EFI partition? That can be deleted and it will be recreated by a new install.
Is it /System/Library/CoreServices/PlatformSupport.plist ? That can be edited with the desired iMac model.
And dosdude1’s macOS Patcher tool works only for older Macs?
 
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K two

macrumors 6502
Dec 6, 2018
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Having once upon a time installed OSX on a Pentium 4 PC, I genuinely find hard to understand how macOS can not be installed on an iMac.
Is it the T2 chip? That can be disabled.
Is it the Firmware folder or Firmware.scap file in the EFI partition? That can be deleted and it will be recreated by a new install.
Is it /System/Library/CoreServices/PlatformSupport.plist ? That can be edited with the desired iMac model.
And dosdude1’s macOS Patcher tool works only for older Macs?
"And @dosdude1’s macOS Patcher tool works only for older Macs?" But one can D/L the most recent full installers from Apple by employing them. 🥳
 

rtcentinc

macrumors newbie
Original poster
Jan 5, 2017
3
0
Sorry for being incognito.. appreciate all the feed back.. here's an update...btw its a 2019 iMac that came with Catalina so all the recovery modes would not downgrade to Mojave.. There is no T2 chip on this model..

To answer the "move ahead" question.. eventually, but not being "forced to" due to the 32 bit cut off.. lots of earlier software versions that would need to be "repurchased".. so yes moving forward is in process but gradually..

I DID get it to install Mojave on the "main" partition--{I say "main", because I partitioned the 2TB fusion drive into 3 separate partitions with the intent of putting 3 separate Mac OS's on it.. Catalina, Mojave, and Snow Leopard}

After hours on phone with Apple and working through numerous methods, it happened.. on one recovery attempt it sent me to Mojave, don't ask how, but it it did.. I think its because I had to erase ALL the APFS drives/partitions that showed up on Utilities- "Show all Devices" View to OS journaled.. at first I did only 2 out of the 3 partitions thinking I could keep Catalina as the main OS and dump the other two OS's on the smaller OS journaled partitions.. but that did not work as noted above.. I did the install of Mojave on the second partition and it worked great and then reinstalled Catalina on the the Main container/partition as the main OS.. both partitions worked "ok". Catalina was a little lagging and would "glitch" a bit-- you could tell it was not running smoothly.. so I decided to to rewipe that main partition and make Mojave the main OS on this 2019 iMac.. its up and running smoothly.. I had a few more calls with the Apple senior techs, they were a little surprised that it was able to work.. but were more than happy to assist..

One issue I'm working through now is, that for some reason the Main drive will not let me make any changes to it unless its "authenticated", which is a pain even time I need to move/delete files... I have worked through all the work arounds on this including terminal and changing the "root" user settings.. Apple Senior techs rans some diagnostics and sent the info to their "engineers".. still waiting for a reply..

Also still trying to get Snow Leopard on that 3rd partition, but that does not seem to be a viable option..{I have some earlier legacy programs it would be cool to keep, and it would kinda be cool to pull this off}.. I have a bootable SL thumb drive, and an actual install CD with an external CD/dvd drive, but she won't boot off either of them, it freezes at the apple or just remains a black screen.. Apple techs seem to think it should work on paper, but of course there is no support on this type of idea...I believe the issue is with the drivers.. wondering if there is a work around on this.. {and of course Apple won't/can't support it, but talking in depth with some of the Senior techs, its kinda a cool idea}.. ;-)
 
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chrfr

macrumors G3
Jul 11, 2009
9,578
3,476
Also still trying to get Snow Leopard on that 3rd partition, but that does not seem to be a viable option..{I have some earlier legacy programs it would be cool to keep, and it would kinda be cool to pull this off}.. I have a bootable SL thumb drive, and an actual install CD with an external CD/dvd drive, but she won't boot off either of them, it freezes at the apple or just remains a black screen.. Apple techs seem to think it should work on paper, but of course there is no support on this type of idea...I believe the issue is with the drivers.. wondering if there is a work around on this.. {and of course Apple won't/can't support it, but talking in depth with some of the Senior techs, its kinda a cool idea}.. ;-)
There is ZERO chance you can install or run Snow Leopard on a 2019 iMac, unless you do so in a virtual machine.
 
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JustAnExpat

macrumors member
Nov 27, 2019
43
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Almost always you can't downgrade the operating system of the computer because of the hardware driver issues. For Snow Leopard, that's not going to work, full stop. You may be able to run it in VMWAre Fusion, or another program like that, but I think the graphic drivers don't work that well though.
 

gilby101

macrumors 6502
Mar 17, 2010
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Tasmania
Also still trying to get Snow Leopard on that 3rd partition
You can't install anything earlier than Mojave 10.14.5 as your boot macOS. Your iMac was first released with a modified version of 10.14.4. Don't waste any of your (and Apple's) time trying to install anything old than that.

My advice is not to try the multiple partition approach for Mojave and Catalina. I don't know why but it seems to cause problems. And in your case doubly so because of the Fusion drive. You will do much better if the second macOS is on an external SSD.

But why do you want both Mojave and Catalina? Is it for some 32-bit apps? In that case, my strong advice is to use Catalina as your boot macOS and to install Mojave (or anything else) in virtual machines using VMware Fusion. This also makes managing your documents, etc. much easier as VMware can transparently link the vm user's folders (~/Documents, etc.) to those on the boot system. Make sure you have sufficient RAM.

For Snow Leopard in particular, VMware Fusion only supports Snow Leopard Server in a virtual machine. Snow Leopard (not Server) is not supported but can be made to work. There is a long thread somewhere in these forums on running Snow Leopard (not Server) in Parallels with a few posts by me regarding VMware Fusion.

Further, I recommend VMware Fusion over Parallels for running macOS client virtual machines - better supported and overall a cleaner experience.
 
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