I have a 5D with three WD NAS 3TB drives. I want to add two more new 3TB drives. I would think it would be best to shut down the Drobo, insert the two new drives then restart it and let it do it’s rebuild. That way it would see both new drives at the same time and proceed accordingly.
Is that okay, or is there some reason they should be added while it is running?
hi str8aero,
usually the process is to add a drive 1 at a time (or to remove 1 for a replacement) while the unit is running, so as not to confuse the unit.
there might be some rarer cases requiring a different approach, though one thing to bear in mind is that if you do not actually need the benefit of 2 extra drives right now, you may wish to add the 2nd new drive at a later stage, because the more drives you put inside, there will be a slight performance hit per extra drive.
(it never affected anything i needed to do, but when someone else kindly advised me to only put in another drive nearer the time it is needed, it seemed to make sense) 
(technically, i think some users have been able to simply add 2 new drives into 2 empty slots before, in quick succession while the unit is on, and the drobo recognised that factor, but if you have a bit of extra time, i would suggest putting in the 1st extra drive into your next empty bay, and waiting for drobo to recognise that fact shortly after, and then if all lights are solid green for all present drives, and the system is stable, you could then add the 2nd extra drive in a similar way.
also this way, by doing 1 drive at a time and waiting a bit, it would also allow any extra volume setups to proceed, without potentially interfering with each other, if your new usable space, was going to exceed your max volume size setting, which would have been first chosen or set-up when you first got the drobo)
Besides posting on this forum I had also sent the question to Drobo Support. They did reply and said I could do either.
With the three 3TB drives I had ~ 3TB of space (with dual disk redundancy) available. I was using about 2 TB.
I decided to shut the unit down, insert the 2 new drives and start it up. I had the dashboard open. The Drobo went through what seemed to be the normal startup - didn’t seem to take much, if any, longer than normal. All 5 lights turned green and once it connected to the Dashboard it showed I had ~ 8TB of space. Seemed too easy - I thought it was going to have to initialize the new drives (and in the end it apparently did) and redistribute data which I though would be indicated and take some time?
So, I thought maybe it wasn’t right. I shut it down again and restarted it to see if it would do anything to indicate it was redistributing. It came right back up, 5 lights all looked fine. Still not convinced that it was really where it was supposed to be, I shut it down again. While it was off I ejected the two new drives and started it up again. Sure enough, this time it went into rebuild, apparently it had been using the two new drives. Obviously I felt a bit stupid…
After ~ 20 minutes the rebuild was complete and the three original drives turned green and my capacity was back to where it was originally. So, then I reinserted one of the new drives and waited. Within less than a minute that drive light turned green and my capacity went up. Then I reinserted the 5th drive (the second new one) and again, after about a minute, that light turned green and capacity went up again. I was back to where I was originally with the two new drives installed.
Seemed way too easy and fool proof, but I guess that is the advantage of the Drobo!
I am curious though - isn’t there some sort of redistribution of data that happened, or will happen? It couldn’t have done it as fast as those lights turned green… I thought maybe it would happen in the background over time - like it didn’t need to tell me it was doing it because my data would still available during the process and it would just happen?
Indeed! Drobo deals with all the complexity so we humans don’t have to.
Relayout will happen over time. Your data was fine before you added those drives. It’s fine now. (With traditional RAID, yes this is a critical stage, but not with BeyondRAID.)
I’m glad it was so easy!
well done for putting your drobo through the test, and that it worked fine
(my hunch is that the redistribution of data, runs in the background for approximately the same duration as a rebuild would take - at least in my recent experience with a 5D - though i believe there is a pending feature request to have this information displayed on dashboard somewhere, to at least let users know that “some process” is still taking place)