Inside a hard drive

From time to time I dispose of an old PC, when I do so I open up the hard drive and take it apart. Both for security and curiosity reasons.

I've noticed in some, though not all, there is a small box inside with a sticky cover. Removing that sticky cover reveals a large number of tiny spheres. First time I encountered this they spilled everywhere, now I'm more cautious . Browsing around the only thing I can think of is that it's part of the drive's air filter.

Anyone actually know what it is?

Desiccant silica gel?

Looks like your right, see link.

While trying to find the answer I came across this video. Running a drive with the cover off!

As has been noted; dessicant to control humidity. There is no air filter because, like a light bulb, the drive is filled with inert gas and hermetically sealed to prevent corrosion and ingress of dust.