this is because when you FIRST got drobo - you picked a volume size (i guess 2TB - the default).
now your physical hard disk space is greater than your volume size, so it has created an additional 2Tb volume so that you can utilise your additional space.
The only way back from this is to wipe drobo and start again with a larger volume size (say 16TB for instance). then drobo would not create a second volume until you had over 16TB of disks in it - something which is not currently possible.
This is called thin provisioning.
DRI do talk about this very clearly in the manual which came with drobo and also on their support pages.
incidentally drobo did NOT ask you to format the drive you had just inserted… it did that automatically and then added it to your storage pool - it was asking you to format the new volume it had automatically created for you so that you could take advantage of the additional space you now have.
i hope this makes things clearer.
the short answer: drobo cannot extended volumes. if your put in more disk space than your current volume - drobo has not make another volume. to avoid this - pick large volumes to begin with!
Sorry i cant be more help :([hr]
this is the article i found on the drobo support site in under 5 seconds of searching for the term “additional volumes”