Most of my music cannot be played in Apple Music App in MacOS ?

"Most of my music cannot be played in Apple Music App in MacOS ? [...]Is there a DIY way to fix this once and for all? On the iPhone, it's also beginning to do this."

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Apple Music Issues on MacStudio:

See if this does the fix:

Go into Music Settings then change the Music Media Folder Location to a different drive path. It then went through the lengthy process of re-organising my music.

Source: Can't Play My Music Files Through Studio Display - Apple Community

"So then, what will happen to all of own music which I did not purchase from Apple, the ones I uploaded from my purchased CDs?: Do I copy them to the new location after the reorg or before ?"

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Export, then Import:

Being your have an Apple Music subscription, ALL music would be accessible between your devices. As for Home music, export your entire Library for safe keeping. Consolidate the entire iTunes home music into a Single Library - that way you can cleanly export them all as one item(a Library). Once exported, restore them to your library when needed. iTunes Store purchases are not included in this backup, because they copy-protected.

You would...

1. Go Here: Back Up and Restore your iTunes library on your PC
2. Consolidate: your iTunes Library
3. Find: the iTunes Media Folder
4. Back up: your Library and Media Files
5. Transfer: the Library to a Pendrive (AKA: USB Flash Drive)
6. Restore: your Library from the Backup to the Other Mac or Device

II. Also, Share these using iCloud:

• Music: Turn on Sync Library with Apple Music - Apple Support
• Movies: Sync Movies Between your Mac and iPhone or iPad - Apple Support

"So then, what will happen to all of own music which I did not purchase from Apple, the ones I uploaded from my purchased CDs? Do I copy them to the new location after the reorg or before ?"

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No. You will NOT have to reorganize your library. Being a Music Library, that means they'll stay organized as they were when you created the backup.