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Before attempting to downgrade you had to go to Recovery Mode and enable External Boot to Allow booting from external media. That way you can install Mojave from a USB drive. I had to Macbook Pro that I upgraded to Catalina and attempted to rollback to Mojave. I was able to rollback on one of them by doing the step previously mentioned. The other had to be sent to Apple for repair. My guess is you will have to do the same.
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Having the exact same issue, and I am very surprised this isn't number one issue people complain about. It is critical for files that are in iDrive (iCloud) to be in a stable and reliable service. I constantly see issues with iCloud folders appearing and disappearing. Then deleting some folders, to have them reappear a few minutes later. Also as mentioned by your post the high cpu usage makes everything slow, worried what it could be doing to my files, which at many times I am not doing any changes. Something as iCloud that had been working fine is totally unreliable now, don’t understand what developers where thinking to improve making it now so unreliable. On a computer with Mojave, things look fine, but whenever I make changes on computer with Catalina, it does not sync changes in iDrive to other computers, sometimes in iOS I see status changed to uploading to some folders. In short, iCloud sync is broken, had to install OneDrive and move all my documents there, just to be safe.
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I took mine on Sunday to an Apple Store, they were going to attempt using the Apple Configurator to restore it, but if it failed, it would be sent to an Apple repair center. I looked the status online, and it looks like Apple repair center has it and working on it.
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For other previous betas, that was possible and easy enough, but Catalina is not letting me restore from backups, now it just enters into internet recovery mode. Internet recovery mode is not working either, boot disk with Mojave is not working, boot disk with Catalina to at least reinstall it, is not working. It is a nightmare.
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Have you figured it out, I have the exact same issue and have an appointment at Apple store until tomorrow.
