How to connect external iTunes library to new iMac?

That's good. It means that selecting that old library should be as simple as our friend, @Niel, has posted. Press and hold the Option key when you launch the Music app.

Use multiple libraries in Music on Mac - Apple Support

If that doesn't seem to be working, please tell us what happens. What is the failure? "It didn't work" doesn't really say much. Are you not presented with the option? Does the library not get seen or displayed?

You might also try booting your Mac in safe mode and see if you can coax the Music app to open the library. boot of your Mac doesn’t fix things, see if the problem still happens in safe mode.  Safe mode will often correct weird software behavior by forcing the OS to clear caches, do disk repair and other self-maintenance and housekeeping and temporarily disables third-party mods.

How to use safe mode on your Mac - Apple Support

Launch Music with the Option key held down, click on Other Library, and choose it.

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Jim-789 wrote:

I tried that but it didn't work. I've also tried going through settings but, again, no luck.

What format is the external drive? Some Apple's libraries like Photos must be on APFS or MacOS Extended formatted volumes, exFAT etc are not supported.

The good news is that function seems to be working. You just have to navigate into the correct folder to find the library file.

You will be looking for a file named Music Library.musiclibrary, or simply Music Library if the suffix is hidden.

I'd start by navigate into the folder iTunes > Music and see if the library file is there.

Is your large library a true iTunes app library that has not been upgraded to a library for Apple's Music app?

What is the macOS version that has been running on the old iMac?