High Humidity will do that even in a PC working 24/7.

Even worse if its a costal area with high humidity as the salt in the air will hasten the corrosion.

Those exposed copper contacts are prone to oxidisation. You can get it off with a mild-hard eraser (for pencils or pens). What migth cause faliure on that pcb is if the interface, headstack and motor contacts get corroded.
Most pre 2000 era drives with more gold coated contacts might be okay. The copper and nichrome stuff are going to "rust" due to humidity, air pollution, chemicals, etc.

I see this a lot on modern HDDs, they seem to use silver coating or something and it tarnishes like that and drive stops working at some point. I have used pencil eraser to revive many drives, once contacts are shiny again things work again until next time in future...

oxidation on copper, but what is that stuff on the solder mask? looks like something is growing, fungus?

Visable on most test points, J2 (Jtag for factory burn in), non-masked vias and screw holes. Looks like damaged around E3 - E12 as it got uner the solder mask. These pcb-s are made to order. WD, Seagate, Hitachi, Toshiba don't make these in house. They all use 3-4 vendors, even in the same family drive, depending on site code and date. It can be considered a quality issue. These consumer electronic stuff would cost 5- 10 cent extra/ unit assembled just so the pcb tracks and pads be more durable.

Like ThrashPanda said if you have salts in the air the chemical reactions can be even more severe.
Just use a pencil eraser on the contacts and store them drives in esd bags with silica desiccant.