I worked in the HDD industry, doing signal processing for a few years. HDDs are pretty much magical. It's insane how delicate, yet robust it all is.

But to put things into perspective with some numbers, the write heads on a modern HDD use somewhere on the order of 50 mA of current. That may sound like a reasonable amount until you consider that the magnetic field [flux] is condensed down to a 60 x 20 nm area. It usually takes a little over 1 Tesla to flip the magnet.

I have been out of the Industry for a little bit now, but things are moving towards a magnetic substrate that has a smaller grain size (allowing smaller bits at a similar SNR) but a coercivity well over 3T at room temperature.