Create old OS boot disk

[DylanFromDurham]

Ok, so after I upgraded to Catalina, a handful of apps I use from time-to-time no longer function. I knew that would happen. It's fine.

As a workaround, I thought I would use an old external drive and install Mojave or High Sierra, and then if I need one of those apps I can boot the Mac using the external drive running the old OS, and then operate these older apps (still stored on the internal drive) in the old OS.

So I downloaded installers for Mojave and High Sierra from the App Store. But in Catalina, these installers won't run. Apple's trying to stop me from installing the old OS over the new one, but what I want to do is install it on an empty external drive.

Solutions?

To be clear, I'm not trying to create a recovery disk, but a full install of the old OS on an external drive.

 

[Lufbrarunner]

With your external drive attached (and correctly formatted) boot with the Command and R keys pressed, this will boot to the OS Recovery, click on Reinstall OS X, click through the screens till you get to the hard disk screen then click on show all disks, select your external disk then click on install. when the OS is finished installing reboot with the option key pressed and you can choose which disk you want to boot from.

Alternatively create a USB installer, Instructions HERE. This shows how to make a Catalina installer but should be the same for other OS's just make sure you select your external disk as the target.