MP 1,1-5,1 MP 5,1 won't boot after wiping SSD for selling purposes.

[AHDW135]

Install Mojave on another drive,

unBless the Catalina bootloader by a deep nvram reset (holding cmd-alt-p-r altogether without any pause until the box chimes 4 times),

set Mojave as startup disk by the control panel,

re-insert Catalina drive,

Mount the Catalina ESP and delete the bootloader (you can use my ESP tools for it as it gives the drive made and type),
(https://forums.macrumors.com/thread...opencore-on-the-mac-pro.2207814/post-32382081)

Now you can install Mojave on the Catalina SSD.


What happened:

I assume you deleted the container of the Catalina SSD. That did _not_ wiped the bootloader. Bootloader fakes the machine is a newer one - but this prevents booting Mojave and give the invalid board fault (forbidden sign) and shuts down after about 30 seconds. But you did not see the sign without a GPU with bootscreen.

Booting somewhere with a black screen and shutting down after a short delay is a typical symptom for the invalid board thing.

I'm not at home right now, will try tomorrow. Just want to make sure about the steps.

1. Remove SSD with corrupt Catalina bootloader
2. Install Mojave on another Drive
3. Set Mojave as startup disk
4. Turn off computer
5. Re-insert Catalina drive.
5. Turn on computer while holding cmd-alt-p-r altogether without any pause until the box chimes 4 times to unBless
6. Mount the Catalina ESP and delete the bootloader using your ESP tools
7. Install Mojave on the Catalina SSD

p.s also not totally sure what you mean by "mount the Catalina ESP"

 

[Macschrauber]

1. Remove SSD with corrupt Catalina bootloader
√
2. Install Mojave on another Drive
√
3. Set Mojave as startup disk
√
4. Turn off computer
√
5. Re-insert Catalina drive.
√
5. Turn on computer while holding cmd-alt-p-r altogether without any pause until the box chimes 4 times to unBless
> don't do this, you might set the bootloader on the Catalina drive as startup again.
6. Mount the Catalina ESP and delete the bootloader using your ESP tools
√
7. Install Mojave on the Catalina SSD
√


If the bootloader on the Catalina drive (might not sure now what kind it is, I assume OpenCore) kicks in again you will get that (with your your GPU, without preBoot graphics, aka bootscreen) invisible incompatible system sign - shutdown thing again when booting Mojave installer or installed OS.

the bootloader lives on an invisible partition of a drive, the so called E)fi S)ystem P)artition.

by mounting it you will present its contents to the Finder/FileSystem. So you can look into it like your other drives and you can delete its contents. Its volume name is usually "EFI".

 

[AHDW135]

Thankyou so much everyone. I figured it out and am now running the Martin Lo OpenCore 0.9.5 version with Monterey. I'm going to keep the computer because it fixed my issue with my TB3 Audio interface I was having before.

I am still running into one issue though. I am running my Mac 4k monitor into my Titan Ridge card as well as my UA interface. If I boot the computer with both plugged in then the UA interface won't connect for some reason. If I unplug the Mac display TB then I am able to get the UA interface connected and then I have to plug back in the Mac Display TB. So close to having this all running smoothly. Any advice on how to have both my audio interface and 4k mac display running smoothly in the same titanridge card?