iTunes keeps deleting all my music on my iPhone
Since about October last year whenever I go to add a new music album to my iPhone 5 (by manually dragging the folder from my hard drive to my phone music collection area in iTunes, as I have ALWAYS done in the past five or so years I have had an iPhone) it deletes all the music on my iPhone (approx. 2000 files) and puts only the new album on my phone. The music that was on the phone is showed as greyed out in iTunes. I can also tell it has been deleted because the storage bar in iTunes drops from 20GB to nothing and the songs are no longer in the Music app of my iPhone. It also deletes all my playlists.
I then have to delete the new album off the phone in iTunes, along with all the now greyed out music that used to work, and then re-add the music in one hit. If I try and do it in batches it just deletes what I have already done. If I add all the music back on to the iPhone and then try and make changes like add album art etc. it also hates that and deletes it all again.
I am using the same laptop I have always used, iTunes is the latest version, and my iOS is the latest version. I have an iPhone 5. I manually sync my music. The music is stored on my laptop's hard drive as mp3 files.
This is driving me nuts! Does anyone have any suggestions/experiencing anything similar? How can I get it so it doesn't delete all my music when I simply want to add another album?
Thank you.
iPhone 5, iOS 8.3