IIRC, HDDs have some reserved sectors in case some go bad. But in practice, once you start having faulty sectors it’s usually a sign that the drive is dying and you should replace it ASAP.

I think if you know drive topology you can technically create partitions on platter level, but I don’t really see a reason why you’d do it. If the drive is dying you need to resilver the entire drive’s content to a new disk anyway.