iMac HDD fan running full speed. What is wrong?
Looks like the drive experiences overheating issue. Unfortunately it's a common issue for Seagate drives which happens with time (had exactly the same problem with iMac 2010 21.5`` and stock 500 GB hdd). I used free Macs Fan Control tool to cool down the HDD (specially in Boot Camp, where the drive was even hotter). The article is about HDD replacement but it also explains how to use the app.

But the best solution I think is to replace the drive with some 2.5`` drive. They are known to be much cooler and quiter (though slower)