I tried upgrading to the newest os on my mac yesterday. Immediately after it completed, my computer refused to start up again. Nothing but reinstalling the old operating system seemed to work, after which I realized it had deleted all the data on my hard drive. I managed to restore most of it from a previous back-up, but my bookmarks and playlists were still missing - not a big problem for most, unless you are a dance instructor who has thousands of songs ordered into playlists, and bookmarks organized for numerous social media sites. I sat down with my dad in the evening and, working through it slowly, managed to find and restore the needed files. Phew!