I recently bought an open-box 2018 Mac Mini to replace a much older one before it dies. I wanted something that would run Mojave since I have some important, orphaned 32-bit software I need to run. Unfortunately (or not) the mini came with Big Sur installed and Recovery won’t go back to anything earlier. My plan now was to install Mojave on a separate partition so Big Sur would still be around to fiddle with. I have been unable to do that.
I used softwareupdate —fetch-full-installer etc. to get a recent, clean copy of the Mojave installer. I then used —createinstallmedia (with and without —download assets) to make a bootable installer on a thumb drive. It actually took me several attempts before it didn’t give an error message (one or more of the BridgeOS packages couldn’t be downloaded.) The resulting installer-on-a-stick boots, the apple logo comes up, and a progress bar starts. Eventually (maybe as long as a half hour) the bar gets to the end and it just sits there. No error messages, no nothing. Note: I did NOT wipe the mac mini’s internal drive before trying to boot the installer. I’m willing to do that if necessary, but only if someone tells me they’ve successfully installed Mojave that way and couldn’t do so without doing the erase.
I took the fetched installer over to my 2012 MacBook Pro and it executed fine and I was able to install Mojave on a partition on an external drive and boot from it (on the MacBook Pro.) I also took the thumb drive bootable installer over to the MacBook and it booted fine; I quit before actually trying to install Mojave, but that at least showed that the installer on the thumb drive appeared to work. So, the installer’s good and the thumb drive is good.
I then tried making a Catalina installer the same way. That worked like a charm and I was able to install it on a separate partition on the mac mini so now I could dual boot into either Big Sur or Catalina.
Whether relevant or not, I tried disabling SIP and Gatekeeper (spctl) on the mini, but that didn’t help. Suggestions? It certainly should be doable since I understand that model Mac mini originally shipped with Mojave. I’m willing to completely wipe the Mac mini’s drive if that’s necessary, and guaranteed not to brick it, but I shouldn’t have to do that, you’d think. There are some possibly relevant threads here that I’ve skimmed, but I didn’t see any that described exactly this problem. I’m pretty Mac savvy, but when it comes to firmware updates and the T2 chip I’m afraid I get lost.
So, let’s restate the question. How do you install Mojave on a 2018 Mac Mini that came with Big Sur? (Either on a separate partition or replacing Big Sur.) You get extra credit if you can tell me why the installer hangs for me at the end of its boot. Has anyone else encountered this problem? I know you can “solve” it by using any of several Virtual Machine packages, such as Parallels, but that would be my last resort.