Hard drives have a reserved System Area (SA) on the platters. The SA is also sometimes referred to as the negative cylinders. This area stores the bulk of the drive's firmware, SMART data, defect lists (G-list and P-list), plus manufacturing test logs.
I expect that there would also be test tracks reserved for read and write testing or calibration. Part of Data LifeGuard's diagnostic testing involves checking the read and write elements of each head. In order to test these components, I would think that the drive would need to write on a "scratch" area within the SA. Perhaps there was a manufacturing oversight that has rendered this area inaccessible???
Are there any differences on the labels of the "good" and "bad" drives, aside from the serial number?