Thank you so much Limnos. This is the best summary of dealing with the iTunes libraries and all of its complex filing systems. Since iTunes is changing every season and MacBooks get outdated every few years, it has often been problematic and I have lost some important playlists in the past, and added mass amounts of music and duplicates that I didn't want.
I also keep my ridiculously large library on an external drive. I simply haven't had the time to begin to sort through the mess and get rid of all the junk I don't want.
When I upgraded to iTunes 12 last week, I made the mistake of not having my hard drive plugged in when I started up the new iTunes. None of my music library or playlists show up in the new iTunes. But, I didn't panic. I knew there was a simple solution to this, although it isn't something you can figure out by simply trying actions in the menu or preferences.
My biggest issue getting the right answers, is trying to find the correct forum discussion, I had to try about twenty different queries and sort through fifty links before I found this thread. Wished I had known this a few years ago !!!! Ever since the iphone, iTunes has become an extension of OSX rather than simply a way to play my music. Do you remember MacAmp? My friend wrote that software back before Apple turned it into iTunes 1.0, and we often just can't believe what it has become.
With your four links describing the key parts to the iTunes filing system, I do think that I may finally be able to do some tasks that I have been too scared to address and take charge of my media again. Thanks Again, and I do hope many others find this thread and end their suffering.