Warnings that my files can not be found while using external hard drive

1 - Changing the media folder location tells Music where new files that music copies or creates will be placed. If you've moved files by hand it won't fix any issues that created. Consolidating the library once links are broken won't help either. See this post Locate multiple missing songs - Music - M… - Apple Community for thoughts on fixing things after the fact.

2 - Yes. In practice life is much easier if each library is located in the folder that contains its media folder. E.g. you might have your main library on the external drive at <External>/Music containing its Music Library.musiclibrary database and it /Media folder. You create other libraries in <External>, e.g. Music 2, Music 3, <Meaningful Library Name>, etc. each of which contain their own /Media subfolders. When the /Media folder is in the same folder as .musiclibrary package for that library then it is easier to backup the library as a unit and move it to a new computer or drive when required.

3 - Sort of. See this post: How can I enable half-star ratings in.Seq… - Apple Community. You have to use a different shell in macOS to use the command, and it only affects behaviour in the Songs layout of the library.

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