I was playing Revelations and had a world in it. One day when I went to open the FTB App, it said that I hadn't actually downloaded the pack when I had never done anything that would delete it. I then went into the files to check, and the entire pack and world are just gone. I didn't manually delete them. Not really sure why this happen; it's pretty frustrating though. I'm on mac and can see similar things have happened to others. Would love for the team to fix this so I can actually play and not worry about losing another save