Desktop icon for drobo changes
There is an easy fix, but it's not so elegant. You can change the default icon used for time machine disks to the icon used for your drobo disk. This can be done by swapping out the system's time machine volume icon located in:

/System/Library/CoreServices/SystemIcons.bundle/Contents/Resources


Note: this will only be an elegant solution if the Drobo is the only time machine disk you use for that machine. Be sure to back up the original Time Machine icon just incase you want it back in the future.