I see that in Final Cut Pro, I can go to File > Library Properties > Storage Locations > Modify Settings and see the following screen:
Set storage locations for the library...
Newly imported or generated media, custom Motion content, cache files, and library backups will be stored in the locations you choose. Existing media, Motion content, and backup files remain in their original locations.
This seems like a great way to create a lightweight backup (e.g. similar to how I had to manually do it in iMovie).
However, I accidentally started editing my project before having this setting enabled. My Final Cut Pro contains duplicates of all the media I imported, and is now Gigabytes larger than I'd like it to be. (Since I already have the original media saved in another location.)
How do I retroactively extract my media from my Final Cut Pro library, in such a way that preserves all the editing I've done?
Update: In Preferences, there also seems to be a setting that affects how media is imported:
Files: Leave files in place (instead of Copy to library storage location)
However, this also appears to only work for new media that is imported rather than retroactively applying to existing media.