MacPro5.1 with BigSur and OpenCore - Want Downgrade to Mojave or 2nd Install

[qwerter18]

Hi,
I just baught a MacPro 5.1 with Big Sur and Open Core allready installed.

I am looking to downgrade Big Sur to Mojave or Catalina, while keeping the Open Core running, to still have the bootscreen available.
I can find so many tutorials how to upgrade, but none how to downgrade.

I am running:
MacPro 4.1 2009 - flashed to 5.1
2 CPUs 3,33 - with 6 /12 Cores
6 x 16 GB RAM
2 SSDs 120GB and 2TB

What would you suggest?

What I allready got working is:
I managed to get Mojave to install on the second SSD via USB Installer.

Now I have 1 SSD with Big Sur
and 1 SSD with Mojave.

I also wanted to keep the OpenCore running, for later upgrades to Catalina or Big Sur.

Would it be possible to downgrade the BigSur SSD to Mojave or Catalina and keep OpenCore as it is, or will OpenCore disappear with the new install (formating the BigSur SSD)?

Is that possible?
Thank you for time and help

 

[blesscheese]

Hi,
Glad to hear you got Mojave installed.

Before you make any drastic changes, what are you trying to run, and why would you want to get rid of the Big Sur install? The reason I ask is, at some point Mojave won't be supported, but Big Sur will still be, and you would just have to re-install it anyway...

 

[qwerter18]

I am planning to use this machine as music production tool.
I am now at the point on deciding which Vsti and plugins in generel I will/should be using. I come from my old iMac/2007 with still OSX 10.11 running. And the only real problem I ran into, were unsopported Plugins on newer MacOS s.
My old Akai Stuff, that shipped with my old hardware or as I read some of the newer Waves Plugins don't work on Big Sur (or even Catalina) yet.
So I try to stay in between the unsupported stuff on the older osx and the unsopported ones on the brand new osx.
Ok... I don't even own Waves Plugins yet, but I also don't know if I will go the UAD or the Slate or the Waves or freeware path....

Actually with a night of sleep, I like the idea of two MacOS running... And testing stuff out.

But I am still curious, how to downgrade a BigSur / Open Core to Catalina or Mojave.
The easy way or at least the easiest way.

Sidenote:
And with Mojave:
I did a createinstallmedia with Mojave installer app, which didn't work.
I did a second attempt with building it via DiskmakerX, and it went through like a charm. But I just learned, that DiskmakerX will not be supporting BigSur any more.

 

[blesscheese]

Well, I can't help you much with your rig, as it seems it has some special stuff.

But, with my Mac Pro, my favorite way of "downgrading" is to just put another drive in and install the desired OS on the new drive. Since you can choose which drive to boot up from, I often switch from one OS to another OS.

That Mac Pro should be able to run every OS from 10.6 (?)-ish on up...

 

[Longkeg]

Would it be possible to downgrade the BigSur SSD to Mojave or Catalina and keep OpenCore as it is, or will OpenCore disappear with the new install (formating the BigSur SSD)?

Hi. Me again. In the world of Macs “downgrade” is a dirty word. Unless someone has come up with a hack I don’t know about the only way to get an older OS on a Mac is to completely wipe the volume that housed the newer OS and do a clean installation of the older version. This would wipe out your Open Core so I don’t think you can do what you propose.

 

[qwerter18]

I see.
Actually I am happy with my setup right now....
Mojave up and running.
Big Sur for testing purposes...

As I come from an iMac, I have to get used to serveral HardDrives with multiple OSs...

 

[basslik]

If I may please. Pretty munch in the same boat that qwerter18 in my dilemma somewhat.

I have Mojave running on a NVMe partitioned drive that shows up as external. The other partition is Mt.Lion.

I have the RX580 working fine without Opencore on Mojave.

When I cloned my Mt. Lion on the free second partition that has GUID, Carbon Cloner says to hold down the SHIFT key so it can loads the files and it will boot normally after file install.

I held the SHIFT KEY down for a long time. no boot ( any help greatly appreciated )