I have a DroboS and previously had it set up with 3x3TB + 2x1TB, with the volume configured for 8TB. One of the 1TB drives failed, just finished the rebuild, and I replaced it with a 3TB. Unfortunately, rather than adding to the same volume as it was before, it is asking me what drive letter I want to assign to a new volume. How come I don’t have the option of joining it to the previous volume as per before? Do I need to replace the other 1TB with a 3TB for symmetry?
hi rob,
if you had those drives, then you might have had the following:
3tb+3tb+3tb+1tb+1tb = 11tb
with single drive redundancy (SDR) it uses up the space of your largest drive for recovery purposes.
so in this case it would be 11tb - 3tb = 8tb
that could be where your max available space for data was 8tb, which was within the max volume limit which sounds like it was set up as 8tb per volume…
if you now replaced a failing drive, thats good ,but because your new drive was larger than the one you had before, eg its about 2tb larger, drobo now works out your max space to be the following, in tb:
3+3+3+2+1 = 12tb
12tb - 3tb (for recovery) = 9tb
max volume size is already set to 8tb
so it now wants to make a new volume/drive letter for you of another (max) 8tb volume, but which actually only will have 1tb of usable space (which you have to remember).
so on that basis i think its ok for you to make a new letter 
hope it helps a bit, and i asked a similar question when i first got mine too.
(mine is setup with 2tb max per volume) 
While Paul’s explanation is perfectly valid, here’s the short version: it’s done exactly what you told it by setting the volume size of 8GB initially. Have you chosen the max value of 16GB back then, you’d be fine, with no additional drive letters now. That’s the whole idea behind thin provisioning.
I just got my 5N how is this volume size set ? I need to buy 16tb of hard drives now to get that volume size?? Please tell me that’s not so. The idea for the drobo was to add as i go???
Put as [many/few] drives of as [big/small] capacity you [need/want/are getting a good deal on]. Set the volume size to 16TB. You’ll be fine. Your OS will “think” it’s dealing with 16TB drive but it doesn’t matter. Drobo will take care of everything and show you the real capacity usage with the blue lights on the front. http://support.drobo.com/app/answers/detail/a_id/452/kw/thin%20provisioning
so how do you Set the volume size to 16TB ? sorry new to all this i’m on a macbook pro and have drobo 5n
Also, what happens to any data on existing Volumes?