Baught old MacPro5.1 with BigSur and OpenCore - Need Downgrade to Mojave

[qwerter18]

Hi,
I just baught a MacPro 5.1 with Big Sur and Open Core allready installed.
I would like to start new, and I would like to downgrade to Mojave.

I am not sure where to post.

I can find so many tutorials how to upgrade, but none how to downgrade.

Thank you guys in advance for your time.

 

[Longkeg]

The MacPro 5.1 came in many configurations over the few years it was being manufactured. We need to know exactly we are dealing with. What processor, how many cores, how much installed RAM? SSD, HDD or Fusion? Year of manufacture? Original MacOS that shipped with it. Was it a server? All of this will help us help you.

From what I can see with the info given Big Sur isn’t supported on this model so someone modified the computer to run it. Not the end of the world but unsupported OSes can be problematic at best.

You won’t be able to go straight to Mojave unless that was the original OS that shipped with this Mac. You may be able to get to it in a roundabout way. You may be able to do a clean installation of the original, older MacOS that shipped with the machine (warning: you will likely
lose all data). From there you should be able to upgrade from that older OS to Mojave.

How to reinstall macOS - Apple Support

You can use macOS Recovery, your computer's built-in recovery system, to reinstall the Mac operating system. Reinstalling macOS doesn't remove your personal data.

support.apple.com

 

[qwerter18]

Ok.
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
The rest I have no idea.

Yes.
It has OpenCore on it and this is running the bootscreen. Grafics is a RX580 with no bootscreen...
All data has been wiped clean and there are 2 SSDs in it.
One with Big Sur
Two with no data.

I built a Bootable Install USB with Mojave on it, but it says,
“This copy of the Install macOS Mojave.app application is damaged, and can’t be used to install macOS.”
So I tried to set the date according to the Mojave Build from 2018, but this didn't work either.

What would you suggest?

I wanted to install Mojave to the second SSD and leave the 120GB with Big Sur untouched.
I also wanted to keep the OpenCore running, for later upgrades to Catalina or Big Sur.

Is that possible?
Thank you for your quick reply!

 

[Longkeg]

MacPro 4.1 2009 - flashed to 5.1

All kinds of red flags and way above my pay grade. Did you do the mods or the guy you bought it from? If it was the seller you should be talking to him.

Is that possible?

Doubtful. There’s a forum around here somewhere for Big Sur running on unsupported Macs. You maybe should try there.

 

[qwerter18]

Ok.
But this Forum is here inside the Macrumors?
I didn't know where to post exactly?

 

[Longkeg]

Yes. It’s in Macrumors but you’ll have to search for it.

 

[Longkeg]

macOS 11 Big Sur on Unsupported Macs Thread

This thread will be used to discuss advancements in getting macOS 11.0 Big Sur running on unsupported Macs. --- Compatibility List: --- FAQ: Q: What does unsupported mean for my old Mac? A: Three problems: Apple locks you out from running the stock installer of Big Sur and as...

forums.macrumors.com

 

[qwerter18]

Thank you! I found it.
But a double thank you, for also posting the link!!!!
Now I have to figure out, how to transfer this Thread over there...
Or just repost...

 

[Longkeg]

Just combine your first two posts then copy/paste to a new post on the new forum.

 

[qwerter18]

All right! I will do that!
Thank you again!