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erase your Catalina system drive.
Reinstall/restore your older system. You can do that from the backup that you were certain to make (before installing an early beta on your main boot drive)
 
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Of course - I can restore from my time Machine Backup - but I need a boot disk don't I?

I turned off Icloud and it seems to be moving properly now.

I hope they sort it out in the next release.

t
 
You should be able to boot/erase/reinstall without the need for a local boot disk - just boot to Internet recovery.
But, it would be a faster method to use a bootable installer for that.
 
Of course - I can restore from my time Machine Backup - but I need a boot disk don't I?
If your TM backup is to a local drive (like a USB drive), you can option key boot to it and get a recovery screen. From there erase the internal drive and restore from a date that still had the previous OS version.
 
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