My son plays Steam games and ever since we'd moved past 10.14 MacOS, some of his games don't work. He found an article that shows we can create a new APFS volume and restore the original version of this machines MacOS to that volume. But our original version is corrupt, so that procedure fails.

Here's where I'm at --

I have a new APFS volume titled MacOS 10.14 created.
I have downloaded a fresh dmg of 10.14 direct from Apple to a flash drive.

My problem is that I don't know how to install that dmg onto that volume. Can anyone here help me with this?

Thanks!

Inside the downloaded DMG is a command line script/program called "createinstallmedia" (perhaps in Install macOS Catalina.app/Contents/Resources/createinstallmedia). Mount the DMG, and run this as root in Terminal and it will install the associated version's MacOS on whatever volume you specify, your new partition, a thumb drive, an external USB drive, etc. Then boot from that volume to run your "old" OS version.

The usual cautions apply, do not inadvertently over-write your working installation, be very careful working as root at the command line, etc. Another way to do this is to copy the DMG to a thumb drive in Disk Utility, and boot from it before doing the install step. This bypasses having to deal with the command line, something many folks find uncomfortable.

There are a number of Internet tutorials on this very subject, with far greater detail specified, some from from Apple Support. The basic process has stayed the same for many years, so older tutorials need not be shunned just because they assume an older MacOS version.

Italics indicate file paths.

I was wrong. It turns out, my DMG was an update only. And Apple won't allow me to download the 10.14 OS. It detects that I'm on a much newer rev and says it's too old a version for me to have. Dang!

Any ideas on how to source that package?

Apple has made the installers for previous systems available due to an expired certificate.

Explanation page that links to all installers (should anyone need).

Mojave page with link.

Hope this helps.

Catalina OS is quite good. Is it possible to leave the Mac updated to the best OS they have, and wait a few days or weeks for the game developers to update them? I thought Steam was an online gaming community that had downloaded games, rather than static DVD or the like.