Drobo-FS appears to reject 4TB drive replacing 3TB drive

After my Drobo-FS marked a 3TB drive as failed (flashing red), I replaced it with a new 4TB drive. The Drobo went through over 24 hours of rebuild, then marked the new drive as red and asks that I replace it since capacity is running low (which is kind of true: my 5x3TB array was down to its last 650GB before that one drive failed). I saw another Drobo-FS user report exactly this problem about two years ago on another forum but there was no reply. Has anyone here encountered this?

My understanding is that Drobo can’t use that extra space until you have a 2nd drive of at least that capacity. If you’re running short of space, how could that extra 1TB be protected otherwise? You can see this in action with the capacity calculator. If you put in 5 drives of 3TB, I see 10.89TB available. If I change one of the drives to 4TB, it remains at 10.89TB. It is only when the 2nd drive of at least 4TB replaces a smaller drive do you get to actually use that extra space.