You might want to initiate a support case with Drobo and submit a log. They can determine which drive initiated the rebuild. And even if you are out of warranty I have seen several anecdotal reports that suggest Drobo will provide "free" log decryption. The log encryption is a very sensitive issue among Drobo users so it makes sense that they might do that.
The only issue I've had with my S was a spontaneous rebuild or two shortly after I got the device, then removed the drives, transported the device and then reinserted the drives. I shut it down and removed and reinserted the drives and never had a repeat. I've seen many reports of something similar, presumably related to not quite firmly seated drives.
I think 24-48 hour rebuilds for the 800i are less than exciting, considering the price. I have not seen any user rebuild reports on the new 5D. I have seen reports of sustained data transfer speeds in excess of 200 MB/s, which is pretty impressive for a Drobo. At $849, a 5D is not cheap in my book but compared to an 800i it is downright cheap .
It appears that our Drobos S may rebuild faster than the 800i, which is counter-intuitive, especially considering the price difference.