You thought correctly. After 90 days you have to go through the manufacturer's warranty service. No retailer is going to take a return after 90 days. In fact, most limit you to 30 days or less for any returns. Costco may have a different policy, but I'd be extremely surprised.
Well, then be extremely surprised as you are incorrect. Costco will take the return under its Customer Satisfaction Policy at ANY time as long as you have your receipt.
Three weeks ago, I replaced a 3 TB HD that had failed after 18 months. I had my receipt, got a new HD and even made almost $30 as the price had changed. I will be replacing another later today that failed after 25 months, but I have my receipt and expect no problems.
Costco is da bomb when it comes to electronics!