FCP is Using Most of My GBs and I'm Afraid to 'Delete' Library Clips etc, in Fear of Losing Files

Deleting generated files is always safe. Deleting original files is not!

Select your library in FCP.

Go to the File menu and choose

Delete Generated Library Files…

Check all thd boxes, and include All Render Files.

Then quit and relaunch FCP.

The point is that all of these generated files can be regenerated later if needed (which is usually never, anyway).

You need to turn off background rendering or the deleted render files will simply re-render.

Open FCP>Settings and choose Playback.

You can then make sure that the checkbox for Background render is deselected.

This does not always work for me, FCP X seems to leak render files and is unable to both delete and use them but the files do take up disk space.

Programs like Arctic can actually delete all render files.... So if the library appears to grow in size without adding new footage this might be someting to consider.