I've been getting a lot of questions about this recently. This has happened to me too. It started some time last year, perhaps just after iTunes 12 was released. I back up my music with Time Machine, as well as with a standard copy, and Time Machine keeps older versions than those in my most recent full-disk backup. I have no idea why iTunes deletes files, and I wish Apple could figure this one out. In the meantime, I'd recommend using Time Machine to make sure you don't lose anything.
Viewing classical albums
Q: I would like to use iTunes for my classical music collection, displaying music sorted by composer. The album, in my case a symphony or string quartet, for example, is then shown with all its "songs" (movements) which takes up too much space. Is there a way to list the albums without displaying the songs?
It sounds like you're using this type of view for your music:
For the albums above, many of which contain short works, it makes sense for each track to be listed. But if you tag your albums per work--for example, a recording of Mahler's 3rd symphony is an album--then this view isn't ideal.
If you do want to sort by composer, then you don't have many choices. You can't change the way iTunes displays content within each of Songs, Artists, Composers, and Genres view. You could use Albums view, which only shows album artwork (or gray squares with musical notes, if your music doesn't have artwork).
But when you click an album to play it, iTunes expands that album and displays its tracks.
You can't always get what you want. Sometimes you have to bend to iTunes' will.
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