ST506 and/or ST412 hard disk drives

[Tony Duell]

>  First, I'm not sure what you mean by ST506 and ST412. I thought
> these were different in that one had a signal used for write precompensation
> while the other used the same line for a head select.
 
Weren't both of those 4 head drives? THe ST412 is part of a family 
(ST406, ST412, ST419) which were 2, 4, 6 head drives from what I can see. 
I thought the ST506 was 4 heads but fewer cylinders.
 
I also thought the main difference between the ST506 and ST412 interface 
was that the latter alloewed buffered seeks, the former didn't
 
I am still curious as to what the 'speacial' signal is.
 
-tony