Maybe different machiens do things in different ways, but I was given an MO Disk that was password locked and they could not get into it.
I put it onto my Atari and when I had a look, sure enough I could not access it, so, I had a look at the password and cleared it with HDTOOLS, gave it him back and he had full access to his disk again.
Now, clearly the tool I used, while on the atari can easily be written for the PC or any other computer just as easily, so I would say that no matter how much you think your data is safe... It clearly is not.