1. I loaded Core Files to secondary drive
I assume that was a "drag and drop
" operation and unless you held the command key (⌘) that was a "Copy" operation so you then had your Core files duplicated on both drives.
2. Then Imported music with preferences in Music App configured to store music files on secondary drive.
Assuming you had Music > Preferences > Files set to "Copy files to Music Media folder when adding to Library" AND
you chose the tunes you had copied to the secondary drive, that would have consolidated those files into the Music media folder on the secondary drive.
If Music > Preferences > Files was NOT
set to "Copy files to Music Media folder when adding to Library" then those files would have remained in place on the secondary drive and would have been indexed
in place. In either case the files on the primary drive would have remained in place and unchanged. OBSERVATION:
Your first step, loading the core files on the secondary drive, was unnecessary. All you had to do was change the location of the Music Media folder in Music > Preferences > Files and import the Tunes from their original location on the Primary drive with "Copy files to Music Media folder when adding to Library" checked
(✓) and Music would have automatically copied your tunes to the new Music Media folder on the secondary drive. 😜 (It took me a while to realize that myself.)CONCLUSION
Once you are satisfied all of your tunes have successfully been copied to the secondary drive you can delete the Music Media folder and tunes on your primary drive. Be aware that Time Machine does not backup your secondary drive so I would advise you to synch Music with your iCloud account as a backup for your music.
Also what does Media : localized mean?
Where are you seeing that?GRUMBLE:
In researching your question I realized my Music Library has gotten into pretty bad shape with lots of duplicates, errors, etc.
so thanks to you I now have a major housekeeping chore facing me. Thanks! 😠