Вот правильный способ сделать:
NSString *chemin = @"fichierlocal";
[[NSFileManager defaultManager] createFileAtPath:chemin contents:nil attributes:nil];
NSFileHandle *handle4write = [NSFileHandle fileHandleForWritingAtPath:chemin];
NSString *lemessage = @"Hello, World!";
[handle4write writeData:[lemessage dataUsingEncoding:NSASCIIStringEncoding]];
[handle4write closeFile];