I'm surprised by the number of people who seem hostile to this question, mostly because they can't imagine any circumstances where they would want to do this. I can think of several. In fact, I was thinking about something similar with optical disks just earlier today. (I'm curious whether small holograms could be created by writing an interference pattern directly onto a CD or DVD, but I would need exact control over how the tracks lined up to achieve this.)

Here's some concepts just off the top of my head:
* He's come up with a new disk encoding scheme, and wants to try it out.
* He's doing research into how long disks retain data, and is questioning basic assumptions like whether surface bits permanently magnetize the platter underneath, affecting later bits in the same location. Or how far the domains spread.
* He's working with self-assembling molecules and needs to give them a patterned magnetic surface to build on.
* He wants to 'print' a 2-D picture onto a small portion of the hard drive, and bounce a laser off it. (The magnetic alignment of the surface domains would polarize the photons slightly.)
* He's making a high-resolution rotational position encoder, so by reading across 32 tracks knows the rotation of the platter down to a few nanoarcminutes.
* He wants to totally destroy the contents of a disk. (I assume this is what most of the hostile people think is the intention, presumably as the payload of some virus)

Those are just the ones off the top of my head.

However, the ranting people do have a point that without knowing WHY you want to do this, we can't really suggest the best solutions. A lot of people have recommended going back to super-old MFM hard drives that allowed this, but we don't know if you require the density of modern hard drives.

To do this with a modern drive, you're basically going to have to rebuild the controller. Either totally remove the controller board (leaving handy raw connections to the stepper motors and drive heads) or cut the connections between the microcontroller and the low-level electrical functions of the drive, and substitute your own. Here's where knowing your accuracy requirements could have help, because if you want relatively large bits, you can get away with fairly low-frequency components. A 20Mhz microcontroller can, with say an external high-speed shift register, push out an 80mbit/s serial stream, which equates to >120,000 bits around the track of a 6,000RPM drive. Not quite the same density as the manufacturer, but better than the old MFM drives.

Your next problem is going to be this: It's really, really hard to tell where the head is on a platter. I've no idea how modern drives do it, but it used to be done with 'marker' bits either in the track, or on a nearby 'index' track, plus a little timing. Preserving these location marker bits is actually the most important job of the drive controller.

It's not impossible, merely very difficult. I could probably make one with only several weeks of hard work.

Alternately, you could try getting into the firmware and re-writing it to your specifications, but that might take longer. You would have to reverse-engineer a lot of stuff that is specifically hardened against this, but at least the hardware would be stable.

C#, Pascal, Javascript??? Sure, and while we're at it why not give him an ADA compiler and the DoD style guide.

Logo was designed and built to teach programming concepts to kids, and excels at it. Has done for 20 years.

I've got a CNC 3-Axis milling machine at home that I don't use much, but we still gave my 6-year old nephew LEGO for Christmas. Think about it.

Ha! I was actually just about to suggest something very, very similar, although with a twist.

I think it wouldn't be too hard to have some junctions which switch based on the ball colour. Effectively being able to sort a whole series of random coloured balls and send all the red ones, for example, to the red ball output slot. That's the clearest demonstration of the primary action of networking: ie, 'routing', I can think of. Not just random paths, but deliberate sorting.

I suppose Ted Stevens was right. It IS a series of tubes!