Driverheaven posted a review on the QNAP 639 Pro NAS System

It is quite incredible how many models QNAP release and how often they are updated but this is due to the relatively cheap prices of large mechanical hard drives and end user demand for ever growing storage space. QNAP were established in Taiwan in 2004 and are well known for their build quality and sophisicated software packages. The QNAP TS-639 Pro is based on the same platform as the TS 509 Pro which we reviewed a while ago and as such now supports iSCSI integration as standard.

QNAP 639 Pro NAS System review