Imac 27" late 2015 Opencore woes

[veryoldman]

Hi
I have a 27" late 2015 iMac. It can run Monterey natively but when I upgraded to Monterey from Mojave about 6 months ago I had so many Bluetooth and performance issues.
I also have an old Macbook Pro 2009 that I used Dosdude to install Mojave on two years ago.

For the iMac I decided to find Opencore Legacy Patcher with the idea to install Ventura instead of Monterey.
Whilst going through the motions of creating an Opencore Ventura installer, I decided to partition the USB drive and also make a Monterey Installer as well. (the reason will become clear).
Both installers were obtained from the OpenCore app links.
So I had the idea of using the Monterey installer as a back-up incase anything went wrong with Ventura (application software incompatibilities etc) but also to first try to overwrite my current Monterey with the Opencore one, just to see if it would correct the current bluetooth/performance issues.

Well everything went ok and the 'new' Monterey completely cured the bluetooth and performance problems ... but I was to get a downer as I ejected the installer USB and shutdown for the day...the next day on booting (without the Opencore installer USB in place) nothing would boot and so I had to insert the installer USB, reboot and then use the 'option' key to bring up the 'bootpicker' selections, select Efiboot, then let the option key off and then choose Macintosh HD. This had to happen on every boot-up.

Now here is the weird part. The late 2015 iMac does not actually need Opencore to run Monterey but I installed the Opencore Efiboot onto the imac so as to get it to boot from the 'new' Opencore provided Monterey system without having to have the installer USB in place. ( I suspect that the Monterey installer was not the pure Apple one and already had some of Opencore additions).
Everything was fine except for a hard drive logo (with Sonoma graphic !!??) appearing on boot up before the apple and progress bar. So I used one of the OpenCore preferences to 'hide' the bootpicker at bootup and only see the white Apple logo and progress bar as per usual.
The 'Sonoma' internal hard drive graphic should not be there as I have not done any Sonoma installing etc.

I thought that I would test just to see if the Ventura install would work but for some reason when I went through the motions I got the message that it could 'not install as the Macintosh cannot be downgraded'. Seeing as Ventura is a newer system than Monterey I am not sure why this would happen. I have a hunch that that strange unexpected Sonoma graphic may be something to do with it..
has anyone had any occurences like this and without wiping the internal drive, how will I get ventura to install when I need to?

 

[Nguyen Duc Hieu]

I guess you will have no problems in creating a Ventura USB Installer using OCLP on your old MacBook Pro.
I also guess that you will have no problem in injecting OCLP EFI to the USB installer.
When you need to install Ventura, just wipe out the internal drive.

For now, I would suggest you repartition (wipe out all existing volumes) the internal drive and start all over again the Monterey installing process. Do a clean install from Internet Recovery function, not upgrading from an old OS.

 

[veryoldman]

I seemed to have cured the issues....without a wipe and clean install.

As I did not want to wipe my drive and have to reinstall all software apps, I had a thought how to overcome my problems with the opencore install issues and only being able to boot up from the USB.

My Method:
I decided to ignore the fact that my late 2015 iMac can already natively run Monterey and I treated it as though it was an older machine that could not run Monterey without Opencore.
I booted from the external usb onto which I had also installed Opencore Legacy Patcher.
I then opened OCLP and went through the process of building/installing the Opencore Legacy Patcher and Monterey (over the one already on the drive) on the internal hard drive.
I then restarted with option held, chose EFi boot at the picker and then option clicked the internal drive, rebooted and... there was now a computer running Monterey much better than the original Monterey install (no bluetooth problems and better wifi and overall performance.

Findings:
So I have found that even a Mac that is able to natively run a version of an OSX Monterey, can also run the same system with Opencore being used.
I cannot say that this is true for machines that can run Ventura (or Sonoma) as my iMac cannot go further than Monterey without using Opencore.
Also as a great bonus, the OSX Monterey installer, used with Opencore process, completely cures the 'Monterey problems' like bluetooth, wifi and performance issues that can arise when originally installing Monterey over the previous system (Mojave in my case).
This was really unexpected and a real bonus.
There is no sign of Opencore working in the background (how much it is doing so I have no idea as my iMac can run Monterey natively). There was no 'extra' Post Install Root patching necessary as obviously my iMac already has all the hardware facility to run Monterey.

There is one Issue:
The only problem now is that sometimes, when waking from sleep, the screen has a completelly solarised distorted and image and the bluetooth mouse is frozen. This does not always happen and seems to be time dependant.
I will test the issue with different settings in the Energy Preferences in System Preferences to see if they affect the issue. The issue is probably due to Opencore elements clashing with drivers or architecture of the Imac. But I can live with it.

Conclusion:
Anyhow, I am now running Monterey as it should have been all that time ago when I tried various times to reinstall it natively to get it to operate properly as it should have.
I do admit that I did not do 'clean' installs of Monterey originally, (obviously without Opencore), so that might have been the reason for the original Monterey installs having issues, but now the 'Opencore Monterey install' has cured it all and I am a happy man.