The password is not a BIOS password at all. It's an ATA password. It's part of the ATA specifications. No data is encrypted on a password protected hard drive. The password hash is written to the firmware tracks on the hard drive itself. When the drive initializes the drive realizes that the drive is password protected and asks the system for the password before it is allowed to access data. Hence why you can't remove the hard drive from the system that created the password and access the data. Most desktop systems aren't provisioned for hard disk passwords so the drives just shut down when they don't get a response to the password.