Cloning macs within a context of no internet access I'm shipping a project which my client (the army) wants to run on new MacMinis2019, which they, being less tech saavy consider a whole (the mac with all the software and configuration installed on it is the product).

These machines are deployed in "Internet-free" zones entirely cut from the internet in order to avoid possible leakages of sensible data.

As part of the many procedures they wish to have access to, one of them is to clone one of the macs onto another one which may be presenting behavioural defects in the software, or whatever... it's no longer doing what they want and the quick and easy fix for them is or rather used to be (with old 32bit MacMinis) to clone a "working" one onto a "broken" one in order to have them all work once again.

I'm having trouble holding up this part of the bargain with the new Macs especially given the no internet constraint.

Here's what I've tried.

I plug in a thunderbolt cable between the two macs, I boot the broken one holding T so it has the thunderbolt logo on screen.

then I boot into the other normally and install and run Carbon Copy which I then use to clone.

The process is error-free and I even sometimes reboot the affected machine to a sucess... once and then on the new reboot it shows me the question mark logo.

sometimes I get this straight away.

I try rebooting holding R to access the disk utility and run First Aid. I then try to select a startup disk from the apple main menu and select the Hard Drive and click "Restart..." but I get a message saying :


  You can't change the startup disk to the selected disk.
  
  An internet connection is required to update this startup disk. Check
  the network connection and try again or select another startup disk.


As I explained above resorting to connecting to the internet is utterly unacceptable for my client.

Why is this happening? is there a better way to clone? is there a command line to type or some other solution to not be prompted for internet connection?
I'm assuming you've disabled SecureBoot, and that the volume name is identical in what follows:

It's possible that the T2 chip on these new (2018) Minis makes 'straight' cloning difficult. 
I had the same problem when I cloned my old 2012 Mini's volume onto my 2018 Mini : I too got a flashing question mark. I was able to fix it by installing Mojave on top of the newly cloned disk. I used a USB install media (after disabling SecureBoot!). The disk booted perfectly after that, with all my files still in place.

I tried blessing the startup volume (though I may not have used the correct commands); and resetting NVRAM, without luck. I can only assume that the T2 chip 'knows' something has been changed and it requires an Apple installer to reset. 

It may be possible to perform the 'reset' without having to do an entire OS install, but I don't know what the procedure might be.