Running Sierra on External HD??

I apologize if this is a SUPER redundant question. I scoured the forums with little luck. I’ll try to be as succinct as possible.

We’ve been running a 2014-era Mac Mini (Sierra) in our Studio B for a few years. No real issues, just a little sluggish with bigger mix sessions. So we purchased a 2018-era Mac Mini (Mojave) with better RAM, processing, etc.

A decent amount of plugins we use are ‘older’ and are no longer updated, and cause kext issues when trying to install on Mojave. We tried a handful of workarounds with no good result. So finally we landed on installing Sierra on an external Samsung SSD. This process went smoothly.

On the old Mac Mini, we have no issue booting up from the internal or external drives. On the new Mac Mini, when we try to boot up from the external drive, we’re greeted with the, ‘A software update is required to use this startup disk’ prompt. I click, ‘Update,’ and it takes me to the, ‘This update will take several minutes to complete and the screen will turn off for up to a minute’ loading screen.

That load screen APPEARS to complete the update, the Mac Mini restarts, and then it brings me straight back to the Update screen. I’m at my wits end trying to sort out why the external HD works without issue on the older Mac Mini, but not the new one. Any suggestions or ideas would be greatly appreciated! Thank you!