This might be pretty simple. Hopefully it is…
Drobo Gen2, connected via FW800.
Disks:
1tb
1tb
2tb
2tb
I replaced the top 1tb drive (wd green) with a 3tb wd red drive and the Drobo is wanting to just format it as a new volume. I was expecting it to just add to what I had, not make a separate volume.
It should recognize drives larger than 2tb, right? Firmware 1.4.2[1.254.50341] and it’s attached to a mid 2011 iMac with OS X 10.9.3 on it.
Dashboard 2.5.2[64171] as well.
If your HDD total capacity exceeds your drobo max volume size (you set that when you first formatted drobo), then it is normal.
What is your drobo volume max size?
You know, that’s a good question. I don’t see it anywhere in the Dashboard, unless it’s…
oh. I think it’s 4 TB. Under “Max Capacity?” Yeah, that’s saying 4 TB.
Ugh. Looks like I’m going to have to reformat everything, huh? 
Yes, if you want to keep your data in one volume then you’ll have to move your data and reformat drobo to a bigger size.
I personally use drobo max available volume size on my drobo 2nd gen and drobo 5N.
It should be noted that on networked Drobo’s (such as the 5N) you can’t manually set the volume size, and it’s always set to the maximum the underlying filesystem supports (16TB on the FS, ungodly large numbers on the 5N).