Before proceding, things to understand:
- Apple unceremoniously and without your consent irrevocably changes your data, so you cannot copy over or clone over a Mail folder or other data like mail, calendar, etc); the data formats will not be compatible. This has been true with every major OS release. I’ve always found that to be highly offensive, but Apple keeps doing it.
- Make a full backup first, preferably two.
Reader Michael D writes:
I bought a new Mac mini 2018 that came shipped with Catalina I was able to roll back to Mojave using this method https://scriptingosx.com/2020/01/downgrading-a-mac-that-shipped-with-catalina-to-mojave/. [MPG: use at your own risk; we have not tested/tried the approach and neither recommend nor dis-recommend those instructions.]
Our accounting software will not work using Catalina, so thus the reason to downgrade. I’m sure other programs are causing issues for your readers.
On the downgrade instructions it says "You could also disable ‘Secure Boot’ entirely, but that is not recommended as it will, well, disable all system security verifications.” I had to do this to get it to work, but I reset the security settings after the downgrade and all is fine afterwards.
MPG: disabling secure boot entirely does NOT disable all system security verifications. But changing Secure Boot to just allow booting off external media should suffice.
Kernel extensions and Secure Boot
Note that disabling Secure Boot entirely is the ONLY way to update kernel extensions in Catalina at this time, e.g., SoftRAID. At present, there exists no way in macOS Catalina to load a newer SoftRAID driver (or other driver) without disabling Secure Boot. Even if you re-enable Secure Boot after installing an updated driver, Catalina will ignore the update.