Hi,
I'm not familiar with iMusic so I don't know exactly what you'll experience, but here are some broad strokes that might help.
*** Experts, please correct me on any of this ***
In the old days:
- Apple offered a service called iTunes Match which did bad things to some folks' music libraries when it first rolled out
- Apple offered a service called iTunes Cloud Library (later renamed to "Cloud Music Library") which also did bad things to some folks' music libraries when it first rolled out
- Apple did a poor job explaining exactly what those services were doing
- Users didn't always understand what those services were actually offering, which led to some bad situations
That's where a lot of the horror stories come from.
These days:
- In both macOS and iOS, Apple provides the Music app
- This is not Apple Music
- Apple Music is Apple's standalone streaming service
- You access Apple Music through the Music app
- Yes, it's confusing
- The Music app can also be a home for your local files
Good things to know:
- Music.app is aimed at people who don't want to deal with files and does everything it can to help them
- Music.app keeps all of its data in a Library
- You can add Apple Music songs, albums, playlists and more to your Library
- You can add local files to your Library
- Music.app will present everything you add to your Library as a single cohesive experience
- If you turn on Music > Preferences > Sync Library, your Library will available on any device you sign into (very helpful)
There are a few things that can trip people up:
- By default, the local Library files are located in Macintosh HD > Users > ***you*** > Music > Music
- You can change this location at any time — This will be important below
- If you add local files to your Library, Music.app will likely try to copy them to your Library
- If you have a hundreds of gigs of music on an external drive and your Library is in the default location on a tiny local drive...you can see the problem
- There's a setting in Preferences to turn that on/off
- Music app has a preference to organize your Library's local files
- If you let it organize, (I think) it will copy all your files to the Library — Same issue as the last bullet
- Also, if you've hand-organized your music files a certain way, you want to turn that option off
- But if your files are a mess, turn it on (it's great for basic organization)
It's worth trying out, but:
- Back up all your local music files first
- Dig through Music.app's Preferences and really understand what it wants to do by default
- Start lightly with just few local files and pay attention to what it does with them
- Get familiar with local song's Get Info > File panel
- Get familiar with the Library's local files
Hope that helps. It's kind of confusing at first but it will make sense eventually. Sort of.
Major caveat: If you're into hi-res music, the Music.app is not bit-perfect. You'll have to manually switch bit depth / sample rate in the Audio MIDI Setup app. One post I read sad you had to restart Music.app as well (I don't know). It's kind of a ridiculous situation now that they're offering hi-res lossless.