Incompatible items on hard drive

[Lifeisabeach]

Yes I do turn it off most nights. And the notice only appears after start up. Here's the actual notice:

Yup. As I suspected. This is not at all what was originally reported and changes everything. So... most likely, the APFS file system was updated when Monterey was installed. When you reverted back to Mojave, the APFS file system was still updated but to a level not fully compatible with Mojave. It's important to understand that APFS is still a fledgling file system and Apple is still working out bugs and whatnot with it. I absolutely would NOT just blow this off. It could be inconsequential. It could be something that could lead to data loss. You will need to make sure you have a secure backup (I recommend two, just to be sure); boot from an external installation; wipe the internal hard drive clean. Don't just erase it. Delete the partitions. I wouldn't even reformat it just yet. Let the macOS installer do that. Reinstall macOS from scratch. Since you are doing this, you could consider going with Monterey. Even though it was slow before, doing a clean install here might fix you up. It's up to you... if it doesn't fix you up right, you'll have to start over with Mojave.

Once you have a clean install of Mojave or Monterey back on the internal hard drive, use the Migration Assistant to migrate your data and apps off your backup.

 

[Rod]

Agreed, I think krs summed up the solution in his post # 17 and it is a bit of trouble to go to but probably the only real fix.
I think it's important to stress that the internal drive needs to be erased at the media level, not the container or volume as in the example below;


That will, as mentioned by Lifeisabeach, remove any partitions.
Secondly, you need to use a macOS installer, not a clone of the old system because that may replicate the problem.

Step one is to download a macOS installer for the OS of your choice, either Mojave or Monterey.
Available here; How to download macOS

Step two is to create a bootable installer on a USB (thumb drive).
Using a free app available here; Make a Bootable USB macOS Installer Download the update for Big Sur compatibility - (you don't need to use their backup software but you will need the updated version for Monterey). Alternatively you can use Terminal for Mojave, see here; How to Make a Bootable Flash Installer of OS X or macOS

Step three is to boot from the USB, use Disk Utility to erase the internal drive then instal the fresh copy of macOS.
You can then restore your data using eg. Migration Assistant as suggested or a Time Machine backup.

 

[garmp]

I'm going to go ahead and try Monterey again. Was thinking about Ventura, but it seems too new & doesn't have any new features that I can't live without.
My now question is do I restart holding the command and R keys to wipe the disk or use Disk Utilities on one of my backups to erase (and partition during/after install somehow)?
During all of this I have a Monterey Installer I am copying to a USB drive. Upon completion of copying I then install Monterey on the USB to make it bootable so I can restart from that?
Then I use Migration Assistant to move my data, etc to the new OS from one of my back up drives and I'm in business?
Sorry for all the basic questions. Just having a hard time keeping up with things.

 

[Rod]

During all of this I have a Monterey Installer I am copying to a USB drive. Upon completion of copying I then install Monterey on the USB to make it bootable so I can restart from that?

No, you will not be able to boot your device from a USB drive with just the macOS on it.
You need to create a Bootable macOS USB installer (Thumb Drive) by employing a 3rd party application like DiskMakerX or Install Disk Creator (macdaddy.io) which erases the Thumb Drive and puts macOS Monterey on to it in a form that allows you to 1. Boot your device. 2. Access Disk Utility to erase / format your HD and 3 instal a fresh copy of Monterey.
PS. I would suggest you save the installer somewhere in case you need it again.