It's been a couple of decades since I posted here.
My large music collection is about 725 GB. Over the years, I've made a copy of it onto a partition of 4 or 5 of the external drives I have. I've had to name the partition the same on each disk. That way, the iTunes library on the internal drive will always be able to find it, no matter which disk is loaded. (Accordingly, the music files (more than 80,000) aren't in the iTunes folder, but are in my own folder hierarchy on the external drives.) I've always had to mount an external drive any time I want to hear any of my music, even if only one song.
That said, I got a new computer with a 1 TB internal SSD, so that I could copy the music there, so that I would be able to listen any time. However, iTunes can't find any of the files unless it's in the exact location I originally put them (and renaming my internal drive isn't an option). I haven't found a way to get iTunes to see those files, when they are still in the same folder hierarchy, but on a differently-named drive.
Perhaps someone can figure out a way to entice iTunes to work, without me having to rebuild my entire iTunes library, which would take several days. I suppose an AppleScript that could read the old library, and then add all the items to a new library. Something like this would take a chunk of time to run, but I don't know whether iTunes is that scriptable.