Something is using my external drive. How do I find out what?

Whenever I attach my external LUKS drive to my laptop, the drive head starts rattling like crazy, as if I am copying the entire thing. Every now and then it pauses.

I've seen some tools like iostat and diskstats but I don't know what they mean, and they don't mention any process or software that does the i/o anyway.

How do I find out what app/process/daemon is making my drive rattle?

-edit-

I haven't found anything using the drive, but interestingly, the same happened when I plugged it into a Windows machine recently. It looks like the drive firmware is doing this, not the operating system? What/why is this? I'm confused.