Running Catalina now - want to dual boot Mojave

[amstel78]

I've been running a patched version of Catalina on my MP 3,1 and it works very well. I do however miss the ability to run certain 32bit applications. So, I attempted to create a dual boot scenario on my Mac.

I downloaded DosDude1's latest Mojave patcher as well as Mojave itself. I also partitioned my boot SSD for the Mojave install. After that was done, I rebooted into a bootable USB drive created using the patcher and proceed with installation on the new Mojave partition on my primary boot SSD.

When all was said and done however, the computer would no longer boot. Just got a question mark and no OS. I ended up restoring my primary partition using a backup created with CCC.

Did I miss a step a long the way?

 

[innovaTutor]

Did I miss a step a long the way?

well, it is complicated. Especially if you mix patched OS versions on Mac Hardware that is missing CPU/GPU Features on which macOS is based on. (this is above my knowledge to explain it accurately! probably @tsialex or @cdf can describe it more technically correct.)

short explanation:
Apple uses different versions of APFS for every macOS. Or let's say it this way: APFS is still a work in progress and every macOS Version knows a bit more of the future. IMHO Catalina APFS is the worst of it and APFS on Big Sur is getting better.

for that reason I advise using a separate SSD for every macOS that needs (its flavour of) APFS.
+ With it comes that every physical Disk has its own EFI System Partition (ESP) where you can install a Boot Manager (e.g. rEFInd) and/or a Bootloader (e.g. OpenCore).

 

[innovaTutor]

I downloaded DosDude1's latest Mojave patcher as well as Mojave itself.

From my own experience on my @dosdude1 adventure:
Those Patcher did only work while he was on it while that macOS version was actually new. As soon as the next macOS came out, he changed focus and sometime older versions got broken after apple patched their older still supported macOS with Security Updates.
He is still a hero IMHO.

________________
little backstory about my knowledge on that topic:
I have been on this catch up adventure since @tiamo wrote a boot.efi for those fake 64-bit Mac Pro Computers that only had a 32-bit EFI. That Patcher was called SFOTT (Sixty Four On Tirthy Two). I finally gave up with SFOTT when the latest Security Update for OS X El Capitan broke my MacPro2,1 System and no solution was found on MacRumors when I needed it most.
I then switched to a MacPro3,1 to run El Capitan.
... and just recently upgraded again to a native MacPro5,1 because I need to get online again (and had some time testing OpenCore).

 

[amstel78]

Thanks for the reply. I ended up solving this issue by using RP/OC instead. I think you're right, and two patched systems can't reside on the same SSD despite either OS residing in its own APFS container.