I don't know the technical side enough to give any real answer, but tossing them in the freezer (properly sealed of course) always seemed to help "loosen" them

I don't know the technical side enough to give any real answer, but tossing them in the freezer (properly sealed of course) always seemed to help "loosen" them up enough to get data off. I've also had some hard drives that you could bring back to life by giving them a firm knock with your knuckles too. Doesn't really answer much, but it's a last ditch effort that has saved me more times than not.