Can I install Mojave on a Catalina computer

[John Rofrano]

[Robert Withers] “I’ve heard that if you have Catalina on a computer you can’t re-install Mojave.”

I believe what you cannot do is to “non-destructively” go backward from Catalina to Mojave, but if you boot your Mac with the Mojave USB stick and you format the internal hard drive, it has no idea what operating system was on there. At that point I don’t see why you couldn’t install Mojave on it.

Here are instructions I found that tell you exactly how:
How to downgrade from MacOS Catalina to Mojave

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com

[Kirk Pitts]

Generally in my experience and, I’ve been installing macs for a school since 1997, you can install the earliest OS that in in use at the time of machine manufacture. Except in the case of new models being made for the upcoming OS. The most recent example was my 2017 5K iMac. The new disk format has made it more difficult.

Kirk Pitts.
video amateur.
Personal skateboard and band historian. 😉

[Patrick Sheppard]

I’ve had very recent experience with this. I was trying to clean install Mojave onto an external drive connected to an iMac that has Catalina installed on it. The iMac was booted from a USB flash drive with High Sierra installed on it. The USB boot drive was necessary because the iMac’s Catalina install was having some issues, which is why I was doing all of this in the first place.

Anyway, it gave me this error message when I tried to install Mojave:

macOS could not be installed on your computer.

An error occurred while updating firmware.
Quit the installer to restart your computer and try again.​

I tried a few times but it wouldn’t work. I think what happened was that Mojave was trying to update firmware that was already updated on the iMac which happened when the iMac was updated to Catalina, and that the firmware installed from the Catalina update was beyond whatever version of firmware came with my Mojave installer, and so Mojave tried to “update” the firmware and failed.

Then when I ran the Catalina installer, it completed without issues and was smooth sailing.

So it seems whether or not it can be done is a bit tricky, and it may depend on what firmware version the Mac has…?