I've been playing the most recent Direwolf20 mod pack for a few days now, and last night whilst playing my client suddenly closed and when I went back on, my world did not appear in the world list. I checked the save folder and the world is still there, I'm not sure what happened but dragging in the level.dat from another world seemed to work, although I did have to use NEI to get my inventory back (luckily I had just set it up to go mining, so I didn't have random things lying in there). I feel that it would be a good idea to use a world backup mod, but I don't know of any for Minecraft 1.6.4. Can anybody help?