Multiple operating systems on a 2019 27" iMac

First: how have you managed to run Mojave on a 2019 iMac? For as far as I know Apple does not support downgrading (and a 2019 machine came with Catalina or Big Sur)

For the 2 operating systems on one disk you should partition your disk

Partition a physical disk in Disk Utility on Mac – Apple Support (UK)

and install the other operating system (for a 2019 machine that can run up to monterey) on the partition you created

WheelieNick wrote:

First: how have you managed to run Mojave on a 2019 iMac? For as far as I know Apple does not support downgrading (and a 2019 machine came with Catalina or Big Sur)

That's incorrect. 2019 5K iMacs originally shipped with macOS 10.14.4.

Thanks for your reply WheelieNick but this iMac came new with Mojave 10.14.4 installed. July 2019, Model Identifier 19.1.

I have read that by adding a volume and installing a later OS on that new volume , I can dual boot.

BDAqua wrote:

I would advise different drives for each, I run Mojave, Catalina, Big Sur, Monterey, & Ventura on my 21" 2019 iMac.

👍 Show off. 😎

I run High Sierra, Catalina, Big Sur, Monterey, & Ventura on separate Macs.

I created a bootable install USB with Monterey OS and installed it on an external hard drive formatted APFS. It boots Monterey OK and runs the apps. I require but in the sharing permissions window, it shows you can only read. When I click on the pad lock, it shows the name I gave it and asks for the password. When I enter the password it will not accept it. I did write it down but now I have no clue. How do I recover the password and when I do, am I on the right track to changing the permissions to read and write.

Which Disk is that?

Catalina & up on APFS split the boot volume int 2, a System Only partition you have no rights to called Macintosh HD, then your user Partition called Macintosh HD Data, which you have limited rights to... could that be what we're seeing?

I would advise different drives for each, I run Mojave, Catalina, Big Sur, Monterey, & Ventura on my 21" 2019 iMac.

Do you have five partitions on your main hard drive or do you have five external hard drives?

All separate drives. Back in 10.0 to 10.13 I had many partitions on one drive, but APFS able to get into an unrepairable state I won't chance many OSes on one drive.