I have a late 27" imac with a 3TB internal drive. It currently is divided into three partitions.. one is running catalina, the second is a carbon copy of the first - the third is my music library. I would like to revert back to the system before catalina - I just don't like it. I understand that the partition holding the main startup system has been changed from HFS+ to something else , kinda dividing it into two. I have plenty of external space, including the previous last-saved setup. My questions are as follows... Can I simply reformat the main partition into the previous setup and carbon copy my old saved setup.. and then use the Carbon Copy backup of the catalina system to boot from, ikn order to update prefs etc?? Do I need to reformat both partitions - or do I need to copy my music partition and just reformat the entire interior hard drive? I AM franticly backing everything up as I go... but just need a little guidance on the "AFS to HFS+ thing...
Going backwards from one OS to a previous will break most databases like Music, Photos, Contacts, Mail since the documents are stored in a database and the structure of those databased are changed in a migration. That migration is one way and unless someone (Apple or otherwise) writes a reverse migration script and tests it and releases / sells it, you have to do all that engineering work for every single type of data you care about.
What's easy is the following:
- Erase your Mac and restore a backup from a time when the backup was on the older OS.
- Erase your Mac and migrate only the apps from the new backup to the old backup. Uninstall / clean any that don't work - then migrate by hand the documents you care about.
- Erase intall your Mac and hand migrate back your data, then install the apps you care about - referring to your backups as a guide if you can't just wait till you need an app, and install it. (seriously - you don't really need things you don't use daily - so don't migrate back a raft of cruft - just start using the new old OS is my best advice - human to human)
- Use iCloud or another service to sync the data - data sync generally works across versions (say you have one Mac on Mojave and another on Catalina - Apple will happily sync photos / music / mail / contacts / reminders across both). Here, instead of having two computers - you have one sync complete - leave the data in the cloud, erase and then sync down the content to the same computer, older OS.
I'm going to stop here since the specifics of what you choose isn't a one-size-fits-all case. If you don't care about anything except files - you could move back pretty easily. If you care about database backed apps - then the list of the apps you care about dictates which operation is best for you.
I would make a good backup you trust now on Catalina - you will need it to try these above steps - and they may fail - you need to at least be able to get back to now to be free to experiment.
I use Time Machine - but the people that wrote and support Carbon Copy Cloner and Super Duper are beyond great. Read their support forums, pay them a tip if you can, many people will have asked about going back and their main answer will likely mirror mine - they are not designed to reverse a one way upgrade, but let's have them answer for their product capabilities.