Desperately need to remove Catalina and reinstall Mojave

I had the same problem and found the solution. A program I use did not work on Catalina after I upgraded from High Sierra. Luckily I had a time machine backup from high sierra although when trying to restore from the Time Machine it gave me the error message. To overcome this I had to go on disk utility and delete both Macintosh and mackintosh HD by using the minus button then the volume reappeared as SSD flash storage I then raised the volume renaming at Macintosh using OS extended (journaled) Then I went back to restore from Time Machine and everything worked out as planned. I hope this helps I’m sure it works the same for operating systems other than High Sierra.

Revert your Mac to a previous macOS version - Apple Support

Ditto on the not a stupid question I am not a computer-minded person, I spent two full days in front of my computer trying to reverse this upgrade without erasing my hard drive since I couldn't trust the backup process I had been using. I was having the same issues with Catalina; heating my machine/fan being constantly on when nothing's open, everything slower, rainbow wheels, had to find a tedious workaround through Terminal to open Photoshop, etc. I came to a lot of dead ends and the thing that worked ended up being super easy, once I manually backed up everything onto a different external drive (in case this process didn't work either). When you're in the disk utility in Restore mode above the listed hard drives is a "View" drop down menu, and I clicked view all, then selected the drive at the very top (my options were Apple SSD Sm25-something-or-another and below that Macintosh HD) and clicked "Erase" I renamed the drive the same thing it was named before, and on the drop down menu below that selected OS Extended Journal. Now I was having trouble with my Time Machine backup because my hard drive was set to the APFS or whatever, and my external drive was set to OS Extended Journal and they are not compatible, meaning my Time Machine backups were useless. By erasing my hard drive and setting it to OS Extended Journal it made my hard drive compatible with my external drive so I was able to hit the "Restore From Time Machine Backup" option in Restore Mode and what do ya know, I'm back in El Capitan and everything is purring. Honestly I'm not sure if I explained the why correctly but I can say that what I did worked. Hope this helps

yeah definitely tried that and it didn’t work. What did end up working though was on the restore screen erasing my hard drive and setting the new parameters to match the backup drive I used, once that was done I got to roll back to a time machine backup that had El Capitan and everything went back to working perfectly. Thanks everybody for the input!

From that disk utility that pops up you need to wipe the Boot Drive “Apple Ssd...” and set up as Mac OS extended journal. Then restart the Mac and you should then be able to restore from time machine back up on external drive.

It should be the Apple SSD SM0512F that you click and use the minus button above to 'erase' - it will then show a window with options to the right - before you actually click to Erase, you want to input the name you want the disk to show after it's erased and right below that field is where you select the OS Extended (Journaled) format. Once you have done that you click Erase.

That was the step I missed the first time. I didn't realize I needed to input the new name/selecting OS Extended format at the same time as erasing - I thought it was a step I did after. You might already know this, but just in case, thought I'd include.

Did you restart in Catalina and try to use disk utility in Catalina (that doesn't work)?

You need to open disk utility from the screen that you get when you try to restore: "On your Mac, choose Apple menu  > Restart. After your Mac restarts (some Mac computers play a startup sound), press and hold the Command and R keys until the Apple logo appears, then release the keys."

From this screen choose disk utility instead of "Restore from a Time Machine Backup."

Thanks so much! It worked. The "minus" button didn't work for me, but I just clicked "erase" and that got me where I needed to be. I restored everything from Time Machine afterwards. It was finished in no time. All seems ok so far. Let's hope it stays that way. This is the first time I've used this forum and I'm so glad I did.

Time machine is a built in back up software that allows you to clone your Mac onto an external drive so if the worst was to happen you can revert to a previous version. If you don’t have a time machine of your Mac from the software you wish to be on then you can create one on the current operating system. Then reset your Mac to factory settings download the operating system you want (eg. High Sierra) then use migration assistant to transfer files and applications. This option however will not let you transfer everything including some settings. You also may run into the issue with some paid softwares and their authorisation keys. I know that’s a lot of info so please ask questions if any of it doesn’t make sense.

I had the same issue! I went back to disk utility and deleted the data volume. Then I went back to time machine and it worked.

Hello almost all correct. Although you don’t need the installer on a separate hard drive. When erasing the internal HD and re naming it you will then be given the option to restore from time machine. Time machine itself holds the version of OS that it was on in the back up so you just need to select the correct TM back up on the drive (i.e. the one from the OS you want to revert to). The Mac will then download the OS from the Apple sever then continue with the restore. Hope that all makes sense let me know if not.

You can download the boot image of what operating system you most prefer from the internet onto a usb drive. Then when restoring to default it should give you the option to boot from external drive.

Sorry for such a delayed response I haven’t checked this thread. You should be able to restore from a time machine back up from recovery. Then if anything is missing use migration assistance.

You will need to erase the top partition so in your case that is the fusion drive. If it does not give you the option right away try mounting it.

You may need to restart your Mac into recovery mode and restore it to factory then try again. When restarting your Mac hold command + R