A unique identification number is stored in the electronic card that is attached to a disk drive, as well as in a reserved location on the disk itself. This ID number is initialized at head-disk assembly and a match between the two ID numbers is necessary for access to the data on the disk, although...http://www.google.de/patents/US20060087760?utm_source=gb-gplus-sharePatent US20060087760 - Simple method of protecting customer data on hard drives returned from the field
