I get a lot of Seagate drives with a bad head. Grenada and Rosewoods are the most common. Can someone explain the mechanics of why a bad head on these drives will read say the first 5% or 10% of the drive without errors (albeit slowly), but after that you get read errors, and the further into the drive you get, the head cannot read at all.

With the drive spinning faster at the start of the drive (outer tracks), in my mind I would have expected the head to read the end of the drive (inner tracks) more easily. But, I guess the angle of the head changes as the HSA moves inward and this is the cause?

I've seen instances where Grenada heads with this problem were cleaned in an ultrasonic cleaner to good effect. I've replicated the process but had no luck. I've inspected these weak heads and cannot see any contamination, so that leads me to believe that the heads aren't contaminated, just 'weak'.

Heads in this state on these models don't come right when I clean them, so swapping in new heads is still the procedure for me. Does anyone have a different approach? More than anything I'm interested in the science behind how they become weak instead of failing altogether, and I guess I've answered my own question of why they read the outer tracks OK, due to the angle being beneficial?

Input would be interesting, thanks.