Catalina won't install on a 2012 MacBook Pro - why?

We recently acquired a mid-2012 13" MacBook Pro that arrived without a drive so we decided to install macOS Catalina onto a new SSD.

We know that the mid-2012 MacBook natively supports macOS Catalina so we fitted the new SSD drive and booted up from our Catalina USB installer.

We used Disk Utility to erase the new SSD drive as APFS format and then started the macOS Catalina install process. The first install phase ran through normally to the point where the MacBook reboots BUT, after a couple of minutes of continuing with the install after the reboot, the MacBook unexpectedly reverted back to the macOS install screen and didn't complete the installation. Trying to boot from the SSD drive then just results in the 'no disc' icon. Mmmmm.

So, what's happening here?

Well it transpires that the previous owner must have not updated their macOS beyond Sierra and had therefore not installed the necessary firmware update to support Catalina.

We therefore needed to install High Sierra in order to get the firmware update applied as part of the install process.

So we booted up from a High Sierra USB installer and found that, as we had formatted the drive as APFS during the unsuccessful Catalina install process we needed to revert the APFS drive back to HFS+ format before proceeding with the High Sierra install.

Completing the install and then letting High Sierra update itself a couple of times via Software Update (updating might not be necessary but we'd like to be sure) ensured the firmware patch was installed.

Then we started up again from our Catalina USB installer, formatted the drive as APFS and then the Catalina install successfully completed.