[jannic]

I guess writing a new hard drive firmware from scratch, without inside knowledge, would be close to impossible.

But why start from scratch if you can just modify the existing firmware? And that seems to be perfectly possible: http://spritesmods.com/?art=hddhack

I'd say the most difficult and resource consuming part is to make versions which work on as many brands, models and revisions as possible, and make them all robust enough so they won't be detected because of random malfunctions.

But I don't know anything about hard disk firmwares: Perhaps they are not too diverse and once you know how to modify one drive, the others will follow easily?