Is it safe to backup ADFS server by Cloning? So if the server crash I will switch on the cloning server.
It might work. There are a lot of caveats though as some of the "clone" might not have the latest data for smart account lockout for example. Cloning isn't usually a supported way to do a proper restore though. You can keep it as a last resort if other ways are failing.
An easy way to backup and restore ADFS is by using ADFS Rapid Restore. See here: https://docs.microsoft.com/en-us/windows-server/identity/ad-fs/operations/ad-fs-rapid-restore-tool
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