I understand what you try to tell me, but that's not what I try to do here.
Of course it's somehow possible to archive (all) the metadata and try to copy it to a new server, but I don't want to archive all metadata.
In a perfect world you would provide a folder with media files to Emby, and Emby automatically gets all the related metadata for it. But of course the world isn't perfect, so there are some files that need manual changes to the metadata. I was looking for a way to do the manual changes in a persistent way.
Currently you can choose between two options.
1) You can tell Emby to store all the metadata inside the media folder. That makes it easy to keep all metadata (even the manually changed data) and to move it to a different server (all data or only part of it). But it adds many additional files to the media folder and therefore I don't like it. Especially as it is not really necessary to archive all the metadata that Emby collected by itself.
2) You can tell Emby to store all metadata somewhere else. This keeps the media folder clean and clear, but it is difficult to keep manual changes if you move part of the media files to a different server.
As I wrote, recently I moved the media files of one library from one server to another. Not all files from the first server and the moved files where added as a new library (beside the already existing libraries) to the second server. As far as I can tell, there is no (easy) way to copy (only) the related metadata as well. So I had to redo all manual changes on the second server.
That made me think of a third option that could be useful: All the automatically retrieved metadata is stored outside the media folder, but manual changes are stored with the media files. That way, only some metadata files are stored inside the media folder, but you can simply move media files to a different / new server and all the manual changes are still there. The media folder wouldn't be filled up with many metadata files, but you could still keep / provide some automatically used own metadata for some media files where necessary.
By reading the Wiki I had the impression, that it is already possible to provide at least your own images with the media files. So I tried it with the image for an artist, but it didn't work. That's why I asked why it didn't work and if there is a different way to provide the image with the media files so that it is used by Emby.