Hello darkforce1

,
thank You for the reply.
Anyway, I am still perplexed.
I cannot understand the "sure" relation between Heads Stack and other parameters (as Configuration Level, Firmware, etc.).
Another fact; about "Seagate" ("U4 ST38421A"), here:
http://hddguru.com/content/en/articles/ ... k-Q-and-A/there is a Table:
"Table 1: The characteristics of donor drive compatibility for head stack replacement"
Regarding to "Seagate", there is written: "+" on "Firmware" and "Country" and a "-" on Part Number; does this mean that Part Number is not important?
My doubt is: can We tell that surely two different (in other Data as Firmware, Part Number, etc.) "Seagate ST38421A" HDDs cannot have the same Heads Stack?
According to the Table, it is necessary to search for the same Firmware and Country, not "Part Number" but I still don'understand; in fact, still If I am correct, Heads Stack (with its IC preamplifier) it's only an analog device (or am I wrong?).
Kind regards

.
Mark