This morning the array was in offline mode. Scary, but on power
cycle it came back up. It is not in rebuild mode any more, so
I can only guess that completed satisfactorily (much sooner than
it had forecast). It reports the new drive as healthy.
sometimes during a rebuild it is possible that dashboard (and/or the computer) can temporarily lose access to the unit. (i am not 100% as to why, and it may just be that in some operations the best way to stabalise that process could be to halt all access until a safer point in time, though usually the units keep working in terms of access when swapping out failed drives etc). in your case though i think you did the best thing which is to try to avoid using the drobo unless needed during a rebuild, as accessing or changing data during one can also slow down the overal usage and rebuild process.
one thing to keep in mind though, is that the estimate values can fluctuate (a bit like windows explorer) though a good value is to try to estimate how long it will take, by using approximately 1day per 1TB of data that you have on the drobo.
if you can remember how much data you had at the time, would that estimate correlate at all?