Delete Generated Library Files in Final Cut Pro
Follow these instructions to delete all previously rendered files from your Library. This can help flush out issues or fix certain problems you may run into.
Select the Library at the top of the sidebar
From the File menu, select "Delete Generated Library Files…"
In the confirmation dialog, check "Delete Render Files" & enable "All"
We may sometimes instruct you to remove Optimized Media or Proxy Media as well. This is dependent on the type of problem you are trying to solve.
Optimized Media refers to versions of your clips that have been re-encoded into ProRes for better performance during editing operations. Proxy Media refers to smaller, more efficient copies of your clips.
Please refer to Apple's documentation for more information.