Mac Mini running Mojave :-
* won’t install Big Sur on an external drive
* can’t be set to boot from an external Big Sur drive
* won’t boot into startup manager to boot from an external drive
I have a Late 2014 Mac Mini running Mojave (10.14.6), which acts as a local server for printing, scanning, HomeBridge, etc. It has been running this version of MacOS for about a year now without any problems. I have not updated it beyond Mojave until now because some of the software it runs has not been available in a 64-bit version until very recently, and until I am certain that the upgrade will not break anything I do not want to do anything to jeopardise its current stability and reliability.
My normal practice for doing an upgrade in such a situation would be to install the new OS on an external SSD, which I can boot from and set up as required. If things go OK I would normally then transfer the content of the external drive to the internal one to complete the update, otherwise I simply revert to booting from the internal drive. This process may involve extra work, but it gives me the absolute security that if all else fails I can quickly revert to the current setup.
However this time I have a problem. The app to install Big Sur is in my Applications folder. Starting the app, it asks for confirmation that installation of Big Sur is required, checks acceptance of Terms and Conditions, and then asks which drive it should install onto. As I understand it, it then copies itself to that drive (takes about 20 minutes), and reboots the system from it to continue the installation process.
In this case, it gets to that reboot, but instead of booting from the external drive it boots from the internal one, and so goes back to the first step of the install app. It therefore never gets to actually perform the installation. This was clearly not going to work!
Next, I took the external SSD, connected it to another Mac (one actually running Big Sur) and installed Big Sur on it there before re-connecting the SSD to the Mac Mini.
On the Mac Mini I went into System Preferences / Startup Drive with a view to setting the external SSD as the startup drive. However, although the drive was visible in the drive list it was not available to be selected; Mojave seems to be unable to see a drive containing Big Sur as bootable.
I shutdown the Mac Mini, and rebooted it with the Option (Alt) key pressed, with a view to booting into the Startup Manager, which would give the option of choosing which drive to boot from. However, the system simply booted straight back into Mohave, without the Startup Manager appearing. This occurred whether I was using a bluetooth keyboard, or a wired Apple Extended keyboard. This Mac Mini simply will not boot into the Startup Manager!
So, the situation is that I cannot use this Mac Mini to install Big Sur onto an external drive, and nor can I boot from an external drive already containing Big Sur - yet I cannot afford to update the internal SSD to Big Sur until I have first tested that all the software I need actually works in Big Sur. As a last resort I may need to physically swap out the internal SSD, but I really don’t want to go down that road unless I am absolutely forced into it!