They make great mirrors though. I used to work in electronic depot repair. We scrapped tons of failed HDDs weekly. We had a guy who's job was to drill into HDD platters all day long (we were required to destroy them to protect customer data).
Some of us as techs woud stick the platters on our work benches to see anyone sneaking up on us. Then I when I became part of management, I had them stuck all over to spy on my techs & as well as those higher ups that were disliked. I've always admired how nicely the parts are machined inside HDDs.
They're also good for scaring birds away if you hang them in trees. I always thought this was a myth but it really does work. Most people use old CDs.