I found this:
"It is not a very well known fact, but all hard disks have a very strong
hardware password capability build in. This password is usually stored both
in a chip on the HD controller (the printed circuit board on the hard disk)
and on the hard disk itself in a special hidden sector.
Setting this password will make the hard disk completely unusable to anyone
that doesn’t know it. And not only on your computer, but on any computer.
[...]
The only way of retrieving any files from a password protected hard disk
without knowing the password is to send it to a data recovery company for
unlocking, but not all data recovery companies could or would unlock a
password protected HD."
http://www.laptoptips.ca/security/hard-disk-password/
