Are the 4TB drives possiby getting pretty hot? External enclusures are not always the best for cooling and 4TB drives are going to have a lot of platters inside which can lead to additional heat. What are the temperatures being reported on each drive? (you can view this and the SMART data error retry reporting for each drive using the HTML Areca tool)
Alternately, it could be a good old fashion case of really bad luck!?
4x4TB are situated in an external Addonics case, spererate PSU and cooling-fans, temp SMART-areca +/- 42.
1x4TB (former external failure) is situated right now in mastercas NLE-hot sweap-check bay, not areca controlled, right now for HD-tune check temp +57, right now NO Ultra DMA failure.
I will put back 1x4TB (former external failure) to the external case and will see, if i will have more "old fashion case" or "bad luck" with 4 drives from totally 5 drives.
thanks Jim, klaus
UPDATE to Jim:
Raid 5 is rebuilding rigt now, Areca-SMART temp of former failured drive + 46.