New 27" iMac - need to downgrade from Catalina

[quatermass]

"I just got my shiny new 27" iMac this morning, and it's a fabulous machine from what I can tell so far. Not so fabulous is that it came with Catalina installed, so half the apps I use daily don't work any more. So, I need to downgrade."

As others have already told you... you can't.

If you (like me) have numerous 32-bit apps that you absolutely want to keep using, you have two choices:
Choice #1:
Return the 2020 iMac while you still can, and get an Apple-refurbished 2019 iMac. These will run Mojave and you'll be able to run your 32 bit apps with a "native boot".
Choice #2:
Use emulation software such as Parallels or VMWare Fusion, and install an older OS that way. You'll still have to BOOT the iMac "in Catalina", however.

Quite... I went with Option 2. Two reasons: No time to swap for another machine - looming deadline, important client, very remote location, so not feasible.
Option 2 is working out quite well. VMWare Fusion 11.5 running Mojave, run the 32bit apps I need in there, shared folders, all good. In fact, I swear it runs a bit quicker than my old Mac Pro 3.1! Almost everything else I've updated or was working anyway, or I can live without for now. Once I have a bit more time I can tidy up the loose ends, but as things stand, I'm happy with it.

Thanks everyone for all the info and advice - it's been invaluable and stopped me wasting a lot of time on stupid things!

 

[r6mile]

I'm not surprised virtualisation on the iMac runs it quicker than native on the 3,1 Mac Pro. In my experience the VMWare Fusion penalty for virtualising another version of macOS is only about 10%, which will still be miles faster than your old Mac Pro.

 

[quatermass]

The person you're replying to is making fun of your typo, in which you typed that the computer had 72 megabytes of RAM.

Indeed... it was absolutely hilarious the first time he did it...

To be really pedantic, it does have 72MB RAM... plus a lot more!

 

[PBG4 Dude]

Indeed... it was absolutely hilarious the first time he did it...

To be really pedantic, it does have 72MB RAM... plus a lot more!

You should probably go to the 2020 iMac memory thread so you can understand the performance you’re giving up by not running your RAM in dual-channel mode. Basically, by mixing RAM sizes (2x32 with 2x4), you give up dual channel functionality. There’s a note in Apple’s 2020 iMac RAM upgrade instructions page that says to use all same size and speed RAM for best performance.

Edit: thread https://forums.macrumors.com/threads/memory-upgrade-troubleshooting-in-2020-27-inch-imacs.2249254/

 

[pldelisle]

You should probably go to the 2020 iMac memory thread so you can understand the performance you’re giving up by not running your RAM in dual-channel mode. Basically, by mixing RAM sizes (2x32 with 2x4), you give up dual channel functionality. There’s a note in Apple’s 2020 iMac RAM upgrade instructions page that says to use all same size and speed RAM for best performance.

Edit: thread https://forums.macrumors.com/threads/memory-upgrade-troubleshooting-in-2020-27-inch-imacs.2249254/

Very true. You need to remove those 2x4 GB to get full RAM speed and dual channel.

 

[quatermass]

Very true. You need to remove those 2x4 GB to get full RAM speed and dual channel.

Thanks for the info - I shall absolutely remove the 8GB and probably get another matching set of 64 to fill the slots. Thanks for the link to the thread too - I shall read that right now! Still very new to the iMac/Catalina world - quite different to Ye Olde Mac Pro world!

 

[Nicole1980]

Very true. You need to remove those 2x4 GB to get full RAM speed and dual channel.

Is there any articles detailing exactly what the performance hit is when you use non matching pairs? I thought I heard it was fairly negligible - like nothing that would jump out at you in real world use.

 

[pldelisle]

Is there any articles detailing exactly what the performance hit is when you use non matching pairs? I thought I heard it was fairly negligible - like nothing that would jump out at you in real world use.

You can find some benchmarks on this forum within 10-25 minutes.

 

[Nicole1980]

You can find some benchmarks on this forum within 10-25 minutes.

Thanks, but I was just wondering if someone could link me to them so I don't have to go fishing around. Whatever.

 

[hoodafoo]

Oh... this is really bad news! Already got the machine, no way to get anything else. Arse!

Just curious, but have you tried to do a full restore from your time machine backup (presumably Mojave)? Or maybe take it to an Apple Store and they have the tools to image it.

I've taken my Mac to the store and they would just plug a cat5 in, you would pick your flavor and in 20mins, it's done.

 

[chrfr]

Just curious, but have you tried to do a full restore from your time machine backup (presumably Mojave)? Or maybe take it to an Apple Store and they have the tools to image it.

I've taken my Mac to the store and they would just plug a cat5 in, you would pick your flavor and in 20mins, it's done.

There is no way whatsoever to make a 2020 iMac boot Mojave.

 

[hoodafoo]

There is no way whatsoever to make a 2020 iMac boot Mojave.

Oh, I totally forgot that the 2020 has a t2 chip! Game over!

 

[gilby101]

Oh, I totally forgot that the 2020 has a t2 chip! Game over!

It is not so much the T2 chip - it is a forward compatibility issue. The last version of Mojave was released in 2019, so Mojave knows nothing about more recent hardware. In the same way my 2019 iMac (no T2) will not run High Sierra or anything older.

 

[hoodafoo]

It is not so much the T2 chip - it is a forward compatibility issue. The last version of Mojave was released in 2019, so Mojave knows nothing about more recent hardware. In the same way my 2019 iMac (no T2) will not run High Sierra or anything older.

You think so? I think it's more about apple being dicks and want to control everything with DRM and signatures. Look at iOS devices where they force you to upgrade then prevent you from downgrading.

 

[chrfr]

You think so? I think it's more about apple being dicks and want to control everything with DRM and signatures.

There's nothing that's changed with regard to OS compatibility with the T2. You've never been able to run an OS on a Mac that's older than what that particular Mac model shipped with. For example, the 27" 2019 iMac can't run anything earlier than 10.14.4, and that computer has no T2, but an iMac Pro, which was introduced in 2017 and does have a T2 can be downgraded to run High Sierra 10.13.2 because that's what the iMac Pro first came with.

 

[hoodafoo]

There's nothing that's changed with regard to OS compatibility with the T2. You've never been able to run an OS on a Mac that's older than what that particular Mac model shipped with. For example, the 27" 2019 iMac can't run anything earlier than 10.14.4, and that computer has no T2, but an iMac Pro, which was introduced in 2017 and does have a T2 can be downgraded to run High Sierra 10.13.2 because that's what the iMac Pro first came with.

I figure the t2 can only strengthen apple's enforcement on OS legislature. The whole block is happening at the boot level. If you can get past that part, I'm convinced you can run pretty much any 10.x on any of apple's intel-based computers

I wonder how the guys in the 2011 iMac modding form are getting Catalina to load on their computers

 

[chrfr]

I wonder how the guys in the 2011 iMac modding form are getting Catalina to load on their computers

That‘s the opposite scenario from running an older operating system on a newer Mac.

This is something which has not changed since the days of the first Macs in the 1980s. The newer Macs have hardware that doesn’t have drivers in the older operating systems, and that does not include the T2. It’s not that Apple includes the drivers in these older operating systems and then just applies a block.

 

[gilby101]

You think so? I think it's more about apple being dicks and want to control everything with DRM and signatures. Look at iOS devices where they force you to upgrade then prevent you from downgrading.

Wrong thread if you want to refer to iOS devices. Macs are different. Don't confuse them.

Apple have never prevented users from downgrading to a compatible Intel version of macOS. And compatible is anything in a range from the first version that shipped with the specific Mac model to the most recent supported by the Mac model. Some examples with 27" iMac models:

2009: 10.6.1 to 10.13.6
2011: 10.6.6 to 10.13.6
2012: 10.8.2 to latest macOS
2017: 10.12.4 to latest macOS
2019: 10.14.4 to latest macOS
2020: 10.15.6 to latest macOS

The workarounds in threads like this https://forums.macrumors.com/threads/macos-11-big-sur-on-unsupported-macs-thread.2242172/ are about getting new macOS on to old hardware. Not the other way round. Limitations getting Catalina and Big Sur onto old Macs mostly involve graphics issues with there being no Metal drivers for old graphics cards.

I'm convinced you can run pretty much any 10.x on any of apple's intel-based computers

If you are "convinced", there is not much we can say. But you are wrong.

 

[hoodafoo]

Wrong thread if you want to refer to iOS devices. Macs are different. Don't confuse them.

We'll see what you have to say about that when AS rolls around

If you are "convinced", there is not much we can say. But you are wrong.

I wasn't speaking to the point whether it's allowed or not. Are you saying that the hardware architecture of the 2019 iMac is SO wildly differently from the 2017 iMac that it's not possible to run 10.12.4 on 2019? give me a ****ing break.

 

[quatermass]

Just curious, but have you tried to do a full restore from your time machine backup (presumably Mojave)? Or maybe take it to an Apple Store and they have the tools to image it.

I've taken my Mac to the store and they would just plug a cat5 in, you would pick your flavor and in 20mins, it's done.

Apple Store not an option I'm afraid. The thing was, I had two illustrations that had to completed to meet a deadline - an exploded view of a Guillotine, and the Acropolis circa 4th Century BC. I had one 95% complete, and the other about 50%. The impending deadlines meant that whatever the solution was going to be, it had to be quick! I initially specced up a Mac Mini on the Apple Store, but the delivery time was 2-4 weeks! So I then found a reseller in Budapest who a) had an iMac in stock, and b) could deliver the next day. I swapped from the Mac Mini to the iMac mostly because of the superior graphics and the 5k screen, and having less cables was definitely appealing!
I thought about trying the Time Machine restore route, but after reading that it would be impossible, and time was getting short, I had get working as soon as possible. Going to an Apple store would take an entire day, with no guarantee of success. Also, my last OS, the one on the Mac Pro 3.1 was El Capitan, so the likelihood of that working was practically nil!
I've managed to find workarounds for most things, and am perfectly happy with the iMac. Getting it set up to my liking, bit by bit. Not keen on Truetone though. Very difficult to produce consistent work if the screen colours are changing all the time, so I've turned that off and recalibrated. Now I have to figure out why the renders from Cinema 4D, which look fine as it does them, look so dark when I open them in Affinity Photo... but that's another story!

 

[quatermass]

We'll see what you have to say about that when AS rolls around



I wasn't speaking to the point whether it's allowed or not. Are you saying that the hardware architecture of the 2019 iMac is SO wildly differently from the 2017 iMac that it's not possible to run 10.12.4 on 2019? give me a ****ing break.

If it's possible, that means someone has done it. So we should be able to see it. If I can see it, and understand how to make my 2020 iMac 27" run Mojave, I'll do it on the spot.

 

[SparkyzFan]

If it's possible, that means someone has done it. So we should be able to see it. If I can see it, and understand how to make my 2020 iMac 27" run Mojave, I'll do it on the spot.

Meee TOOO! I am in same boat with 32 bit. I just got a new MacBook Air (Catalina) with i7 and 16GB ram. I was not aware that I could not downgrade from original OS before I bought it. A VM seems to be solution to most of my problems. The main program I am hanging on to is Adobe InDesign b/c I don't want to pay for cloud type apps. Anybody tried running Adobe InDesign with either VMware Fusion or Parallels?

 

[quatermass]

Meee TOOO! I am in same boat with 32 bit. I just got a new MacBook Air (Catalina) with i7 and 16GB ram. I was not aware that I could not downgrade from original OS before I bought it. A VM seems to be solution to most of my problems. The main program I am hanging on to is Adobe InDesign b/c I don't want to pay for cloud type apps. Anybody tried running Adobe InDesign with either VMware Fusion or Parallels?

I'm running VMWare Fusion 12 with Mojave as the VM OS. Photoshop CS6 works pretty well, though it doesn't like moving whole layers about in an A3 400dpi image!
Other than that, no probs - can't see why InDesign wouldn't work fine.