NikKTech has one of Seagate's IronWolf Pro 12TB hard drives on the test bench today.
With the first 12TB drives already available in the market by pretty much all the major players (including Seagate, WD and HGST) and 14TB models already on the way and scheduled for release by the end of the year (if all goes well that is) it's a safe bet that mechanical drives aren't going anywhere for many years to come.
