Can I downgrade from Catalina to Mojave and back up my files if I only have a Time Machine back up from after I upgraded?

I understand. It will probably work, at least for an older version of Logic Pro X. Since it wouldn't even launch in Catalina, it couldn't have altered your work files.

The bare essentials are:

• boot macOS Recovery using the appropriate key chord to reinstall its originally installed operating system
• use Disk Utility to completely erase the startup disk
• install that operating system (which must be Mojave or earlier)
• upgrade it to Mojave if that's not what it is
• When asked if you want to migrate information, do that. Select the Time Machine backup.

If you need help finding the applicable Apple Support documents let me know.

I'm not a Logic Pro X subject matter expert, so consider soliciting more qualified assistance here: https://discussions.apple.com/community/professional_applications/logic_pro

Yes, using the "get Mojave from the App Store" link found on this page: Upgrade to macOS Mojave - Official Apple Support.

Just be advised there have been sporadic reports of Mojave "not being available" for a variety of reasons. You can try that download now, or at any time.

The installer app will appear in Applications and will be backed up with Time Machine just like everything else—files, apps, etc, but it won't work to install macOS unless the Mac is using an earlier macOS version—the reason the startup disk has to be erased first.

For next time:

It will be tedious, but what you will need to do is completely erase that Mac and reinstall its originally installed operating system. Then, upgrade that operating system to Mojave, and when asked if you want to transfer information from a Time Machine backup, do that.

Some of that content will not work—anything that required an update or modification to comply with Catalina's requirements for example.

Catalina has caused me nothing but problems.

You might be better off explaining those problems. They might be fixable. If you relied upon 32-bit apps, there's no way to make them work in Catalina though... you'd need to update them, which usually requires payment.

Your Catalina photos library will be unreadable under Mojave. A lot of people have 000's of photos. You need to export them all to a USB drive as individual photos while Catalina is still installed. Then import them into Mojave photos and reorganise them into Albums etc manually.

There is a reason one performs a last Time Machine backup prior to an operating system upgrade. It gives you a clean reinstallation of the prior operating system with everything precisely as it was before the upgrade.

Sure, you can perform a clean install of Mojave (if you can find it), and yet, when you attempt to restore from a Catalina Time Machine backup, it will attempt to restore Catalina-specific pollution back into Mojave that may introduce data and .plists that Mojave's Finder, and other Mojave specific applications cannot handle where there have been unique Catalina changes.

I don't believe you can pull this off without penalty or profanity.

I don't believe you can pull this off without penalty or profanity.

Or both.

Me and my conjunctions. 🤓