Can NOT downgrade on iMac with Preinstalled Catalina

[rtcentinc]

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]

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.

 

[Juicy Box]

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.
[automerge]1582783044[/automerge]
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:

How to Downgrade a New Mac to Mojave from Catalina - TidBITS

If you’re looking at buying a new Mac now, it will likely ship with macOS 10.15 Catalina. If you’re not ready to upgrade from Mojave, it’s still possible to downgrade most new Macs, but it might not be easy.

tidbits.com

You can give it a try:

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.

 

[Fishrrman]

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.

 

[Riwam]

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]

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]

...or you could run catalina, update when updates come... and move forward (instead of backward). just a thought!

 

[Juicy Box]

...or you could run catalina, update when updates come... and move forward (instead of backward). just a thought!

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.

 

[fisherking]

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]

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?

 

[Fishrrman]

Haven't heard back from the OP...

 

[K two]

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. ?

 

[bogdanw]

If everything else fails: OpenCore on the Mac Pro https://forums.macrumors.com/threads/opencore-on-the-mac-pro.2207814/

This guide explains how to use the excellent OpenCore (OC) bootloader ...to install, run, and update macOS Catalina, resulting in a clean, unpatched operating system no different than what you would find on a supported Mac.

 

[rtcentinc]

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}.. ;-)

 

[chrfr]

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.

 

[JustAnExpat]

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]

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.

 

[K two]

?

 

[NewsView]

Re: K two,

I came across the above screen shot of the many macOS/X operating systems you were able to install at the conclusion of this thread and attempted to message you but it wouldn't go through (some error about "appears to be spam"). As a result, I am replying directly to this thread even though it was started in 2020.

I have been using Macs since the early 1990s but have fallen extremely out of date (still using a late 2009 Core 2 Duo 27" iMac (10,1), mid-2012 MacBook Pro (9,1). As a result, I failed to appreciate before I placed an order for a deeply discounted new 27" 2020 iMac (MHJY3LL/A, which is apparently an upgraded configuration of MXWV2LL/A) that I will have no way to run Microsoft Office 11 and CS6 web & design applications on Catalina, which apparently comes preinstalled..

Before stumbling across your posts on this thread, I scoured the Apple discussion forums and elsewhere for answers but it wasn't until I saw your screen shot that I had any hope that there may be a workaround that will allow an older macOS to install/boot.

I am somewhat hesitant to go the emulation route because graphics acceleration is apparently unsupported. (Having never worked with VMware Fusion or Parallels I am also concerned it could be slower than my current setup). So before I buy a license for Parallels or VMware Fusion, I thought I would reply to inquire if the methods discussed in this thread might work for a 2020 iMac that also shipped with Catalina? If so, is there another thread here — or a guide — that describes how to pull this off?

Thank you. I appreciate your input.

 

[chrfr]

I am somewhat hesitant to go the emulation route because graphics acceleration is apparently unsupported. (Having never worked with VMware Fusion or Parallels I am also concerned it could be slower than my current setup). So before I buy a license for Parallels or VMware Fusion, I thought I would reply to inquire if the methods discussed in this thread might work for a 2020 iMac that also shipped with Catalina? If so, is there another thread here — or a guide — that describes how to pull this off?

Thank you. I appreciate your input.

As covered earlier in the thread, there is no way to install an operating system older than Catalina on a 2020 iMac. A virtual machine would be the only option.

 

[NewsView]

As covered earlier in the thread, there is no way to install an operating system older than Catalina on a 2020 iMac. A virtual machine would be the only option.

That's long what my understanding has been too — until I saw K two's screen shot in post #18 and post #10 from bogdanw.

K two's startup choices range from 10.6.8 to 10.14.6. I could be wrong but I'm not aware of any Mac that natively supported Snow Leopard but can also run Yucca Valley. Unless I'm mistaking that screen shot (from within a emulator?), it suggests that there are workarounds for macOS versions that predate the hardware.

Years ago I configured my existing setup on a partitioned drive containing Snow Leopard (native for the 2009 iMac) and a second partition for El Capitan. I had hoped to duplicate a similar setup this time with CS6 web & design and MS Office 11 running in a Mojave (or High Sierra) partition and the newest macOS running in another partition for improved security, web browsing and the like.

Optimistically, I would like to believe that the 27" i9 3.6Ghz 2020 iMac I just purchased will be such big upgrade from my existing hardware that even in emulation without the benefit of graphics acceleration it will beat the performance of CS6 web & design on my 2009 iMac and 2012MBP. But if that's asking too much — or if I am unable to obtain a license/support for what are apparently "depreciated" versions of VMware or Parallels that will run Mojave, High Sierra or similar — I may have to revisit my assumption that a 2020 iMac will do the job better than a 2019 or 2018 model (purchased used).

My original idea, before coming across a Prime Day deal for a NIB 2020 27" iMac, was to replace my 2009 with a similar vintage Mac mini for legacy apps, buy an Apple Studio Display and a new M2 MacMini and swap between them on an as-needed basis. However, that idea became complicated rather quickly given that an old MacMini isn't going to have the graphics card support to drive a 5K Apple Studio Display. However, after ordering the 2020 iMac, I belatedly came to appreciate that apart from the Catalina limitation on 32-bit apps, I will lose other things like Target Display Mode and Target Disk Mode. As a result, I am trying to nail down my options on the 2020 iMac before it is delivered on Wednesday. If there's really no good option (can't duplicate what bogdanw or K Two have apparently done) and/or VMware or Parallels isn't a good fit for CS6 (can expect slower performance), I may very well return the 2020 and buy a slightly older 27" iMac on OWC for about the same price.

 

[chrfr]

That's long what my understanding has been too — until I saw K two's screen shot in post #18 and post #10 from bogdanw.

K two's startup choices range from 10.6.8 to 10.14.6. I could be wrong but I'm not aware of any Mac that natively supported Snow Leopard but can also run Yucca Valley. Unless I'm mistaking that screen shot (from within a emulator?), it suggests that there are workarounds for macOS versions that predate the hardware.

OpenCore is used to allow newer operating systems on older hardware. It won't work in the other direction. Yucca Valley is not the name of a macOS version, but it shows 10.14.6, which is Mojave. A 2010 or 2012 Mac Pro can natively run both of these.
You will likely find that Adobe apps are effectively unusable without graphics acceleration in a virtual machine.

 

[JustAnExpat]

*MAYBEEEE* it's possible. The only time it is possible to downgrade is if Apple releases a computer, and then they release an update, and the computer comes automatically with that new upgrade. The computer can be downgraded back to the version it should have came with.

Further back than that? No.

 

[greyeyezz]

Interesting. 2020 iMac, preinstalled with Catalina. Installed Monterey then upgraded to Ventura. All is fine but I need to run some older software and printer drivers that only work with Cat. I'll just install Catalina on an external. Wrong. When I try to install from a Cat thumb drive in recovery I get stuck in a reboot loop. Reinstall Catalina doesn't show up any longer just Ventura.

IMG_20230909_151516529 by https://www.flickr.com/photos/146780422@N05/

 

[MacGeekYYC]

Just a thought, possible workaround - sign up for the personal use version of VMWare Fusion 13 Player and install a small Catalina virtual machine for the older software and printer drivers?