Ultrastar drives are known for reliability. Still, it's better to know if your drive is still in warranty or not. Now here's the problem: HGST drives have a completely different set of serial numbers compared to regular WD drives, and apparently WD hasn't made any update on their RMA status page regarding HGST drives. Fortunately, HGST has a page to do that, and it just works very differently compared to WD's. Here's how you do it:
1. Go to https://www2.hgst.com/portal/site/en/support/warranty/
2. Enter your SN (either check the sticker or use HDTune to see it) together with the captcha (it's annoying AF).
3. In select Country/Region, try your own region first, and click 'Check Warranty'. If the site says the SN is invalid, go back and change the region to United States. If that still doesn't work, keep changing the region and try again before calling WD or your retailer.
In my case I succeeded after choosing US as my region. Hopefully this will work for you too.