Hmm. No antivirus. Could it be Steam's fault? I'll try and boot the game with nothing else running. Update in a sec.
No luck booting KSP by itself. This weird. Nothing else is running not Steam not anything.
Same problem in the log file:
IOException: Sharing violation on path /Volumes/DAWNWALL/Games/Steam Library/steamapps/common/Kerbal Space Program/PartDatabase.cfg
at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] in <filename unknown>:0
at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share) [0x00000] in <filename unknown>:0
at System.IO.File.Open (System.String path, FileMode mode) [0x00000] in <filename unknown>:0
at ConfigNode.Save (System.String fileFullName, System.String header) [0x00000] in <filename unknown>:0
at ConfigNode.Save (System.String fileFullName) [0x00000] in <filename unknown>:0
at PartLoader.SavePartDatabase () [0x00000] in <filename unknown>:0
at PartLoader+<CompileAll>c__Iterator49.MoveNext () [0x00000] in <filename unknown>:0
(Filename: Line: -1)