Thank you for answering me.
First, to your question: No, there was no file in the library before restoring from the backup.
And, to be sure, I redid the whole process from a new clean install of Media Center.
- Make a new backup of the library on the old Mac
- Move my Music folder
- Restore the library on the new Mac from backup
- Change the file path with the Find and Replace tool
- Run Rebuild Thumbnail tool (a long enough operation)
I got the same results as the first time, all the files were restored, except that a lot of the metadata are lost.
In fact, everything I personally added or modified:
- Some missing cover art
- Some changes of names, titles, artists, etc.
And to go further, I started all over again with the difference that:
- I did not restore the library from a backup
- I only have fully transfer the J River folder in the Mac library: "/ Users / iMac-01 / Library / Application Support / J River"
And here, instantly, I find the entirety of my library with all the additions and changes I made personally.
So I come to believe that there is something wrong with the Backup and Restore process.
I also conclude that not all metadata are embedded in the tracks files, but that some are rather preserved in the MC library.
To confirm this, I only had to open my Music folder with another application (Swinsian) and also have the track files analyzed by a specialized application (Yate).