Thanks for the clear answer...
but {quote}writes the metadata into the file header (in case of JPGs, PSDs, TIFFs){quote} so it DOES re-write to your files. I appreciate its only the header, but that does leave scope for corruption if LR misbehaves, and possible updates to the various timestamps. I may be fussy but I don't want "anything* messing with my original files, not even the EXIF/ITCP headers. Does it not backup before doing this overwrite?