if i understand “single” drive redundancy correctly, a rough plan of 4x2TB drives, means you have approx 3x2TB of usable space. eg you lose your largest drive to redundancy/security etc. = approx 6TB usable.
(Usually you also lose a bit more for normal formatting etc. so its probably more like 5.5TB usable or similar)
Now if i understand “dual” drive redundancy correctly, a rough plan of 4x2TB drives, means you have approx 2x2TB of usable space. (eg losing your 2 largest drives worth, to redundancy/security etc). = approx 4TB usable.
(if the usual formattating takes up a bit more space, that probably leaves you with around 3.8TB usable).
exact figures will be more accurate via that drobolator calculator but these are just approximations.
In this example i was hoping to be able to point out that 1.9TB cant fit twice into 3.8TB but im now puzzled myself LOL -