I have to admit, when I first read about missing songs in iTunes, I was confused. I was 100% sure that this had to be user error. This was based on my experience with a 19,000+ music library. Some of which I ripped from CDs, some I downloaded from somewhere other than iTunes, and some which I purchased from iTunes. I’ve never had any problems with losing songs. I keep my master iTunes collection on a Mac mini at my house, and access music via iTunes Match on an iPhone, iPad, and work MacBook Pro. I occasionally add music on my work MacBook, and then use iTunes Match to download the songs on my Mac mini at home.
Something changed recently, which lead to me loosing some of music, and I think I know what caused this to happen. The first sign that problems were starting showed up in movies streaming from my iTunes library to my TV. Sometimes videos would “glitch” and the audio would get out of sync, making the movie / tv show unwatchable. I was pulling my hair out trying to figure out why this was happening, but never found a solution. Fast forward a few months, and I started to have major issues with a number of my attached drives on my Mac mini. It started with a non-repairable HFS directory on a Drobo. I moved all of the data off of the Drobo, and then the 4TB drive that held my iTunes library starting having IO errors, so I backed everything up to the newly emptied Drobo. After everything was copied, I tried one more time to repair the directory on the 4TB drive, and it seemed to work. After about a week, it seemed that even though the directory could be repaired, the drive was still having issues, so I re-coppied the iTunes library, and reformatted the drive. This is when I noticed the missing files. The dreaded exclamation marks started show up. Thankfully, I also had a BackBlaze backup, so I was able to recover some of the Apple Lossless files that were missing. The rest of the library was able to be recovered from iTunes match.
At this point, I’ve given up on the non iTMS purchased movies and TV shows that were on that drive. It will be easier to re-rip them than to figure out which are corrupt.
TL;DR: Songs went missing from my iTunes library, and it’s likely a result of my hard drive being corrupt.