Current conclusion: if you are using cameras that encode a log profile, for which FCPX automatically applies a LUT, you will have to copy media into the library. It cannot be external. It works fine externally until you go to move or back-up the library. Then automatic log processing is lost in the back-up and cannot be recovered. So, before backing up or moving, you have to consolidate the files so everything ends up inside of the library.
Since FCPX uses symbolic link (sym-links), the media isn't actually copied into the library during consolidation. The library file size is reported as if it grew by the amount of imported/copied media, but the drive capacity will not have changed and the "copying" process is nearly instant. However, when you do copy to a back-up drive, at that time media is actually placed into the library that is being copied and the dupe will in fact consume the amount of space it says.
Unfortunately this means that if you need access to the media from multiple applications, you will end up duplicating media, but I don't see any other way around that, until this bug gets fixed.