I have recently created a laminar hood that I can open and service hard drives. I have opened and successfully changed the heads on hard drives and have one waiting for a platter swap to a donor hard drive with a good motor. The heads have had extended contact with the platter due to a motor malfunction the results are a large swath of platters (all 4) with magnetic material scorn off. The bright line close to the hub and outer band about 1/4 inch on the outer rim (are BAD)
I removed the heads and replaced with donor heads I doubted would work after seeing the damage.
looking at the heads through a microscope showed that every platter has been damaged
My next rout is to change the platters to another good working motor.