Configure your Mac Mini to boot from a Different Partition
You may know all this already, but in case not, here is how to set up a Mac-Mini to boot from different partitions. Ultimately, you would do the same if you want to set up a bootable partition from an external harddrive.
When your Mac-Mini setup has been completed, apple-id / icloud setup included - you would go to Disk Utility to partition your harddisk.
Select your Macintosh HD volumes and click on the Partition button in the toolbar of Disk Utility.
Click on the little "+" plus sign under the pie-chart
Make sure you select ADD PARTITION
Enter the size in GB for the new partition - I created a boot-0 for which I then made a clone of a fresh Mac OS install For that purpose, 40GB was enough.
You can also select your size using the pie-chart tool there.
Give the new partition a name and click the APPLY button.
Wait for the re-sizing to be over.
Then download and install the Carbon Copy Cloner (CCC) program. https://bombich.com/download
Start a new task (cmd+N)
Select Source - your Macintosh HD partition
Select Destination - your newly created boot-0 partition
Click on the little Hard Disk Icon in the Destination Section and a pop down menu will appear
Select "Legacy Bootable Copy Assistant" and click on the "Allow CCC to erase 'boot-0' ..." BUTTON and CLOSE
Press START to start the cloning process.
When CCC has finished, go to your Mac - Preferences (Apple Icon left Top - menu item- "System Preferences")
Select the "Startup Disk" icon the last icon / bottom right
Click the lock to make changes
Select the "boot-0" icon and wait a little for the system to make the change. When done, the dialog will refresh with the boot-0 icon highlighted.
re-start your mac.