RN312 - Upgrading disk size

I do not know where you read about any drive incompatibilities.  Your NAS can handle at least 12TB drives with no issue.  Specs you see are based on the largest drive available at the time of release.  There may be a realistic point at which your NAS processor is too slow to handle bigger drives without a serious performance issue, but we aren't there yet and the actual OS limit is apparently well beyond what spinning hard drives are likely to ever achieve.  Many of the newer, larger drives have an alternate mounting hole pattern that will not match the tooless inserts of your NAS and only match two of the four screw holes, but two is really enough so long as you don't ship it with the drives installed or do something else that really shakes it.