I hate to ask this...

[Vbob]

The newest MODEL Macs I’ve worked with are 2018 Mac Minis - these have the T2 chip, and when we bought them they came with macOS 10.14 Mojave.  
I had occasion to upgrade one to macOS 10.15 Catalina, and THEN had to go back to Mojave - so that is possible...

Some tricks with the T2 Macs - they are delivered with several boot options disabled.  
To get around that, you need to create the first admin account in a normal startup, then boot into Recovery mode, and run the Startup Security Utility <https://support.apple.com/en-us/HT208198> to allow for external booting, and/or non-original OS booting.


In my experience [92 Macs of various vintage at art school…] Target Mode is unreliable now & Disk Utility->Restore is broken by design. 

Carbon Copy Cloner still works, is under current development, Mike Bombich is a saint…  there is a generous trial mode, but send the man some money!

If you’ve been saving installers untested - um, Apple’s own certificate expired last fall, and many of them won’t run. Ask me how I know.

Drill down on this page <https://support.apple.com/en-us/HT210190> and you should be able to get a currently-certified Mojave installer. Apple tries to convince you not to use it, but then offer a link near the end.

Our own IT people say none of this will work next year….. but I think that’s because they’re going to purchase all our Mac direct from Apple already enrolled in some MDM scheme, and make us learn JAMF to install only what they say we can..

YMMV - good luck.

Rule 1 - backups!
Rule 1.5 - use Carbon Copy Cloner to make a bootable copy of your successfully set up machine before you break it - I use 120Gb SSDs and a cheap SATA-USB dongle <https://www.amazon.com/BENFEI-Drive-Adapter-Cable-Compatible/dp/B07F7WDZGT/ref=sr_1_1_sspa?crid=XSIY77CKU07F&keywords=sata+usb+cable&qid=1581275949&sprefix=sata+usb,aps,145&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExQUhOUzhSOUpYQzRRJmVuY3J5cHRlZElkPUEwOTMwNzI2TjRUV1BLQkNIR1dRJmVuY3J5cHRlZEFkSWQ9QTAyNDY2MzQzVlJFMUlJMk9BQTRMJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==> as a minimal bootable external drive to walk about with, since our network is wonky….

Long live the older Macs!

Vbob



> On 2020.02.09, at 1:27 PM, James Sentman <james at sentman.com> wrote:
> 
> This is true, they changed something about the lower level formatting of the drive. In order to go backwards from Catalina I believe that it is necessary to reformat the drive from scratch. I haven’t tried this but that seems to be the consensus out there. If you reboot the new machine into target disk mode you should definitely be able to load it with a carbon copy cloner image file saved from a good, pre-catalina maching. CCC will alert you to any issues while installing it and should offer to do any reformatting necessary. At least I hope so, again, I have not yet had to do this.
> 
> Dee Dee, and anyone else in this situation. I’ll be hopefully resuming my earlier attempt at providing a truncated but otherwise working version of XTension for catalina by providing a properly notorized binary distribution. If you run the non-notorized version then Apple will relocate XTension to a temporary folder to keep it from doing theoretically bad things (that as far as I know have never EVER resulted in an exploit of any kind, but still, hey fix all those things that never happened and break the stuff that everyone uses every day, thats just a great idea Apple…) like linking in a dynamic library that got snuck into the distribution folder but which was not signed by the developer. Which is something else I’m not sure how I would even go about doing if I wanted to but do admit that it should be at lest possible…
> 
> Once you have the properly notarized application you should be able to run the fewer but otherwise functional plugins. 
> 
> You can also go back in time in the releases on the wiki and find the last release version for which I was able to also do the notarization. I’m not sure what that was… Let me see if I can make them cough up a notarized copy of the version I’ve got here now, hang on ;) If I can get them to admit that XTension is not malware I’ll post a link to the pre-build minus the non-catalina compatible plugins as soon as they come back with my signing key for it. 
> 
> 
> 
>> On Feb 9, 2020, at 12:52 PM, Ed <eds-ha at ealkenn.net <mailto:eds-ha at ealkenn.net>> wrote:
>> 
>> Recently, however, I read (I think on Reddit) that Apple has made it nearly impossible to go backward in OS versions through changes in the firmware that ships with new Mac models. Has anyone tried to install an older OS from a thumb drive on a <NEW> mac?
>> 
>> 
> 
> Thanks,
>  James
> 
> 
> James Sentman                       http://www.PlanetaryGear.org <http://www.planetarygear.org/>		http://MacHomeAutomation.com <http://machomeautomation.com/>
> 
> 
> 
> 
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