If Final Cut Pro projects aren't rendered after updating to macOS Catalina
When you first open an existing Final Cut Pro project after updating to macOS Catalina, Final Cut Pro might start rendering again or might not show video thumbnails or audio waveforms. The size of your Final Cut Pro libraries might also be larger after updating.
If a project needs to be rendered again, the render indicator (a light gray dotted line) appears below the ruler at the top of the timeline in Final Cut Pro. If Background Render is turned on in Final Cut Pro > Preferences > Playback, you might notice your project begins rendering in the background a few seconds after you stop working and moving the pointer in Final Cut Pro.
If a project starts rendering after updating to macOS Catalina, you can remove unused render files from your library to save space on your disk.
If Background Render is turned off, you'll have to manually render again after updating to macOS Catalina.
Remove unused render files
You can remove older render files from the whole library to save space on your disk:
In the Libraries sidebar in Final Cut Pro, select a library.
Choose File > Delete Generated Library Files.
In the window that appears, select Delete Render Files.
Select Unused Only, then click OK.
Unused render files can also accumulate for any of the following reasons:
Changes made to a project timeline
Updating Final Cut Pro
Updating macOS
Updating third-party plug-ins
Moving a library between macOS versions
Render your project
If a project isn't rendering automatically, you can manually render a project. With the project open, choose Modify > Render All (or press Control-Shift-R).
Final Cut Pro renders the files in your project and removes the corresponding dotted render indicator. You can view the progress of rendering in the Background Tasks window.
