Dedicated servo
An older scheme where a whole platter surface carried nothing but positioning tracks, followed by a head of its own while the other surfaces stored data.
Before embedded servo, high-end drives gave over a whole platter surface to servo tracks. A dedicated head followed that reference, and the other heads — rigidly ganged to it — tracked in parallel. It gave continuous feedback but assumed every surface stayed perfectly aligned with the reference, an assumption thermal expansion broke as track density rose: the data heads drifted relative to the servo head. It also threw away a whole surface's capacity. Embedded servo, which reads position on the same surfaces that hold data, made it obsolete through the 1990s. A few drives ran a hybrid of both.