What to Do When a macOS Update Goes Wrong
When you’re ready, in most cases the best thing to try next is to start up in Safe mode, which is explained in full here We often don’t do that properly, and it’s worth following Apple’s instructions very carefully[…]
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If Safe mode makes little or no difference, the next thing to try is downloading a standalone version of the update and installing that.
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In Catalina, one unique problem which can occur is that spurious additional volumes are created by the installer. To discover whether that has happened, check your boot disk using Disk Utility.
Previously:
What is the solution, if the installer has done so so (created suprious additional volumes?)