Now the waiting truly begins. fingers crossed against additional failures. On the next upgrade i’ll be able to test to see if the Drobo Pro (the original Drobo 8 bay) supports disks greater than 4TB. Apparently, no one has tested that. I honestly wonder how many of these are still in use? I have now replaced every drive in my Pro.
Update: I have been informed by @mistacabage that the Drobo Pro will not accept drives larger then 4TB. The official documentation just said it was untested.