How to downgrade macOS High Sierra to macOS Sierra

Hi,

Use Carbon Copy Cloner or Time Machine to backup your Mac to an external hard drive > erase the internal drive, format it as Mac OS Extended (Journaled) GUID > Go to the macOS Sierra page > click on Open on the left side of the page (or Install if there’s an Install button) > click on Continue and follow the prompts > restore from the backup.

Question: Didn't the original poster need to create the Carbon Copy Cloner or Time Machine backup before they had upgraded to High Sierra? My understanding is HS has a different file system and is a bit more complicated to revert back to Sierra. Thanks for the clarification.

Read the following for more information: Everything you need to know about Carbon Copy Cloner and APFS | Carbon Copy Cloner | Bombich Software

My understanding is that the method I posted will work because restoring will only restore the data/files/settings, etc. not the OS or format (note that I did not say to clone back using CCC, as that would just put back exactly what was there before, HS).

Not unless you have another Mac and use it in Target Disk Mode to manually transfer your files, then manually transfer them back, install all your apps again, and reset all your settings.

You can get a new HD for $50-$60, then you'll also have it to use for backups afterwards, which everyone should be doing.

One caveat I didn't realize: "Keep in mind that when you open an Apple application on the newer OS (e.g. Mail, Contacts, Calendar, etc.), those applications will immediately and irreversibly upgrade the user data for those applications. You cannot simply reinstall Sierra (for example), then go about your day with the upgraded user data; the Sierra versions of those Apple applications can't use the upgraded data. If you need to downgrade to a previous OS, it is imperative that you have a complete, bootable backup of your Mac as it was prior to the upgrade."

tjk wrote:

Not unless you have another Mac and use it in Target Disk Mode to manually transfer your files, then manually transfer them back, install all your apps again, and reset all your settings.

Either way, it appears you'll lose the data from all Apple apps, such as Mail, Contacts, Calendar, etc.

Yes, this was my concern. It seems a full data backup in the original file system needs to be made before starting the High Sierra upgrade. Otherwise we might have data in the new file system format that is not reversible and readable back to Sierra. Thank you tjk for the updates here. That was helpful.

Your concern is certainly warranted. Here's a video from CCC that explains what should be done prior to the upgrade, which will then allow you to painlessly revert to a previous OS with everything intact. Hopefully it will help someone safely prepare before they do the upgrade. Note that a backup should always be done even if the old OS won't be downgraded in the future; a backup is always needed in case something goes wrong, whether you're upgrading the OS or just running normally.

If you have to downgrade, here's what you need to do

Video: Downgrading from High Sierra to Sierra using a CCC bootable backup

It would be really helpful if the Apple instructions for upgrade from Sierra to High Sierra made it clear regarding a full backup of data (maybe it does?). In this case, is a Time Machine backup sufficient to support full reversion to Sierra from High Sierra if that is needed (including the Apple app data for Mail, Contact, Calendar, etc that would be restored from the backup done BEFORE the High Sierra upgrade). I understand in this case we would lose changes to data from those apps that were stored only in the new APFS format. I'm not sure users really understand the possible issues here and the best way to reduce the risk of a problem.

Agreed, but they do say "Before upgrading, it's a good idea to back up your Mac." How to download macOS High Sierra - Apple Support.

Time Machine will restore your computer back to Sierra. You won't have any data added since the backup was made in Sierra.

Time Machine – Revert to Previous OS