My Mac mini (i7/1TB/8gb) arrived this week and came preinstalled with 10.15.3. In order to boot from my Mojave bootable usb stick, I had to turn off security on the secure boot screen / macOS Recovery.
Secure Boot = No Security
I couldn’t specify to boot from the usb stick as it was giving me the “the bless tool was unable to set the current disk” so I ended going back into secure boot screen and changing the external boot to allow. I didn’t read the second option, probably too excited.
External Boot = Allow
Use Startup Security Utility to make sure your Mac always starts up from your designated startup disk and always from a legitimate, trusted operating system.
support.apple.com
The Mac mini is running on Mojave 10.14.6 without the 2020-002 and Safari 13.1 update, reading other discussions about 2020-002 worries me so I’m skipping it for now. The usb stick was a standard Mojave installer made on my MacBook Pro also running 10.14.6. Took about 10/15 mins to install Mojave.
Now I’m waiting for the TR6 screwdriver to arrive to upgrade the ram.
Update: Installed the 2020-002 Mojave security update from the Apple download site rather than from Software Update within System Preferences. I've kinda always done that, then applied the update with the existing Apple 8GB RAM installed. System rebooted a few times then came back on after a few reboots and 5 minutes later. Then did a TimeMachine backup. Then upgraded the ram with the purchased 2x16gb sticks carefully and successfully. Mac Mini is back up and running and all seems fine.
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