間違い
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentationDirectory, NSUserDomainMask, YES);
NSString *dir = [_paths objectAtIndex:0];
FMDatabase *db = [FMDatabase databaseWithPath:[_dir stringByAppendingPathComponent:@"file.db"]];
正解
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *dir = [_paths objectAtIndex:0];
FMDatabase *db = [FMDatabase databaseWithPath:[_dir stringByAppendingPathComponent:@"file.db"]];
「NSDocumentDirectory」が「NSDocumentationDirectory」になっていました。