diskutil and df show different sizes
You can't format a Drobo with Disk Utility. You need to use the Drobo tools - 
see this tech note

Key part:


  Although some utilities (such as Apple's Disk Utility in OS X 10.5 and
  later) allow you to repartition or dynamically repartition a volume
  without losing data, Drobo device volumes do not support these
  utilities. Drobo storage device volumes should not be repartitioned
  using any application other than Drobo Dashboard. If you repartition a
  Drobo device drive using a third-party utility (including OS
  utilities), you risk losing the data on the drive.