A typical HDD design consists of a spindle that holds flat circular disks, called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminum alloy, glass, or ceramic. They are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection.[46][47][48] For reference, a standard piece of copy paper is 0.07–0.18 mm (70,000–180,000 nm)[49] thick.

Source: https://en.m.wikipedia.org/wiki/Hard_disk_drive

This comment suggests it may contain leaded solder.