I know It has been a couple of months since someone posted something here.
I'm absolutely new to unRAID, I bought a used Areca ARC-1680IX-24 and connected it to 6X2TB seagate in a Norco RPC-4220.
I had problems with smartmontools not getting any SMART readings from the drives.
I managed to track down this in the areca FAQ:
http://faq.areca.com.tw/index.php?sid=308038〈=en&action=artikel&cat=9&id=85&artlang=en
My controller had version 1.48 and after updating to the latest firmware (1.51) it works just fine.
I thought it's worth mentioning in order to help anyone who's having this problem,
and I can also confirm that this method works for 1680 controllers as well.