my take here is seller fail to prove his innocence

any attempt to twist with change mobo, firmware etc is invalid since seller fail to show proof that the HDD is a different unit, its one & the same, & its faulty

he wanted to skirt the rule & escape his obligations for a refund

the faulty item has been returned to seller thus he should refund & case can be close

based on seller's way of replying here, I doubt he will honor it, so TS just take it as a lesson to scrutinize seller's credibility more carefully next time

even if lets say the HDD became faulty due to shock after rough handling / fall during transport; the seller is still liable & should send a working unit as replacement, otherwise refund the buyer