Western Digital Corporation offers 12 TB capacity versions of their "WD Red" and "WD Red Pro". In most places on the globe this actually went on sale a week or three ago. 

The SATA 3.0 (6 Gbps) HDD comes at 12TB "WD120 EFAX" and "WD Red" and would serve medium sized NAS servers quite well. 1 to 8TB are spinning at 5,400 rpm class with advertised transfer speeds of  196 MB/sec, cache capacity is 256MB at 5,400rpm. The spec of the 12TB model "WD121 KFBX" of "WD Red Pro" is better though, 7200 rpm class, transfer speeds at 240 MB/sec, cache capacity is 256 MB.

We talked about these a few weeks ago, but now they are actually available at a 500 USD/EUR marker. They are tagged under SKU codes he WD120EAFAX and WD121KFBX.