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...

  • You don't need to go back to HFS, Mojave wants to run on APFS too. Cloning from Catalina back to Mojave though is another thing entirely. Generally going 'downhill' is a whole whole lot tougher than cloning like to like. You'll probably get only 'documents'. Such as mail, apps & iTunes won't translate properly. You can try it, as you've a safety copy, but I don't hold much hope tbh.
    – Tetsujin
    Commented Sep 20, 2020 at 15:35
  • You have to erase your Mac to go back, so I'll set aside all the format / etc. When you erase the Mac, follow Apple's steps and erase the entire drive - don't mess with partitions / volumes unless you have lots of time to learn. Do the sure thing is my advice. Also, don't be frantic backing up - you have time to plan this right - getting the backup correct now - complete now is your primary goal. Once that's done, you can experiment and learn.
    – bmike
    Commented Sep 20, 2020 at 16:13
  • Which OS was installed in the iMac initially? You can‘t go back to an earlier version, might be better to use a VM then
    – nohillside
    Commented Sep 20, 2020 at 16:23

1 Answer 1


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:

  1. Erase your Mac and restore a backup from a time when the backup was on the older OS.
  2. 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.
  3. 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)
  4. 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.

  • I do realise that much will need to be transferred from the current catalina to a previous OS but luckily, I don't use the databases in music/iTunes, or photos... and i don't use apple mail. My question is simply whether or not I have to erase the second partition on my internal drive, where i keep the carbon copy clone of the "main drive/partition... or can i keep it, and restart from it, in order to get prefs and such after I have erased the first catalina partition and installed mojave.. or some other earlier system.?
    – mufngruf
    Commented Sep 21, 2020 at 9:30
  • Wow, I missed you did the clone to the same drive. @mufngruf There’s probably no reason you couldn't erase just the OS and Data partitions and leave the backup alone, the installer runs fine (heck I often run two os side by side, so if there’s room you could make a Mojave container and install there before you erase Catalina. The only downside is 1) all your eggs are in one basket. 2) confusion or mistake on naming or selection.
    – bmike
    Commented Sep 21, 2020 at 11:48
  • thanks - f Here's my plan... I shall do an uptodate CCC to partition 2, from partition 1 (my current catalina startup system.) I then propose to restart my computer, using this clone partition 2 and Carbon Copy a previous saved drive onto the partition. Will doing so automatically then convert that partition back to HFS+ and allow for the copy to be installed - just like it once was and, having done that, will my computer boot into either drive partition .. using either system.. thus theoretically enabling me to simply copy over prefs and stuff? I really do appreciate all your help. tony
    – mufngruf
    Commented Sep 21, 2020 at 12:12

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