I guess you are having drive failure issues w/ the ST31000528AS model instead of the ST31000340AS. We use both models & the 528AS series seems to be less reliable than the 340AS. We have both drobo & droboPro units & so far out of those 1TB drives (around 30 units) that we have deployed only 1 drive (i.e. 528AS) failed - certified dead. Neither our drobo & droboPro & Windows XP PC failed to recognize it. In ur case, the chances of two 1TB drives failures in a month is uncommon, I recommend you should contact DRI support cuz I’m unaware of the drobo or the Pro comes w/ diagnostic tools.
Yes I have had multiple Seagate 4TB drive failures - two have failed within 2 weeks (not long after firmware update) Drobo support say that it is a HD problem - Seagate say their drives are error free (after long diagnostics). The drives were Seagate 4TB Barracuda DOM both NOV2017. I am now £200 out of pocket
!! I would be very reluctant to fit Seagate drives to Drobo unit again.
I personally never use Seagate drives…too many issues. But if your drives are using Shingled magnetic recording (SMR) as a recording technology, the excessively slow writing process will cause definite issues in any RAID-type storage box.