This should help to explain:
If Disk Space Exceeds Volume Size
If you installed a drive in Drobo and you are now being prompted to format, this is because the total storage offered by the inserted drive(s) now exceeds your volume size. The Drobo Dashboard will automatically create a second volume which appears as another volume icon on the Mac’s Desktop or as a new drive letter or partition on the PC. You will be asked to format and name the new volume. Formatting the second volume/partition will not affect the first.
If you are using a DroboPro or DroboElite, you will be prompted by Drobo Dashboard to create another volume through the Volume Management facility.
Current model Drobo’s support 32TB & 64TB Volume and/or Storage size. For more information, see “Maximum Volume and Largest Size Drive”
Some legacy Drobo models allowed you to format your Drobo device’s volume size as 1TB, 2TB, 4TB, 8TB or 16TB (even without having that much disk space). This ability to format your volume to be larger than available disk space is called Thin Provisioning. Thin Provisioning allows you to grow into a single large volume over time rather than needing multiple smaller volumes. However, keep in mind some older operating systems have a 2TB per-volume limit.
Note: Even though your operating system may prompt you to format your volume, always use Drobo Dashboard for formatting. Drobo Dashboard allows you to format your volume to a size larger than the available disk space (thin provisioning).
Maintaining a Single Volume
If you have formatted your Drobo legacy product for a smaller volume size and now wish to have one single volume you will need to backup your data onto another Drobo or storage device, reformat the original Drobo device to the volume size of your choice, and then move the data back. Many customers choose the largest capacity available during initial setup to avoid having to move such large amounts of data around.