This thread jogged my memory on the subject, see the
following link
on how hard this can be.
http://sodoityourself.com/hacking-ibm-thinkpad-bios-password/
This is for the BIOS password, well different from the ATA (HD)
password.
As far as I know, the ATA password is unbreakable. It is stored on the
platters AND the controller. So if you change controllers it would not work.