How to Downgrade a New Mac to Mojave From Catalina
Apple has started shipping Mac models that used to come with Mojave pre-installed with Catalina. If your organization has blockers for Catalina (incompatible software, etc.) you may want to install Mojave on these Macs. Unfortunately, this is not so easy.
[…]
Directly downgrading from Catalina to Mojave with the
startosinstall --eraseinstallcommand will fail. Attempts to run the Mojave installer from a Catalina Recovery (local or Internet) will also fail. The reason seems to be that the Mojave Installer application chokes on some aspect of Catalina APFS.[…]
The solution requires a Mojave Installer USB disk.
[…]
To boot a new Mac with a T2 chip off an external drive, you need to allow external boot from the Security Utility in the Recovery partition.
Via Josh Centers:
We’re still hesitant to recommend upgrading to Catalina, particularly if you use Mail, since reports of email data loss continue.
I continue to receive reports of Mail messages lost when upgrading/migrating/rebuilding and when moving messages between mailboxes (particularly between different accounts). There also seem to be a bunch of new crashing bugs affecting Exchange users, as well as problems with messages not moving (but, thankfully, not disappearing either). Yet many customers are also using Catalina without these issues. It’s not yet clear why some are affected and others aren’t, but it’s good to know that downgrading is possible (unless you have a 16-inch MacBook Pro or a 2019 Mac Pro).
Previously:
