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Obituallist: Is there a level.dat file in the backup? Use that. Any level.dat in fact would work to "restore" access to the world, but the seed will be different so any terrain subsequently generated will not seam properly. Creating a new game, with the same name, should create a new level.dat in the existing folder and it will pick up the existing region data files so your build will be preserved. Of course, the new level.dat will have a new seed so...
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rhn: What generates level.dat_old?
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Obituallist: What do you mean it won't save anymore? This makes it sound like one of the region data files has become corrupt. If you want to try and save your world, go into the "region" folder of your save game (if you can find your level.dat you can find this) and open each of the .mca files therein with a zip/archive tool. Delete the .mca that does not open.