Drobo Pro with 6TB Green Drive - really slow writing

I added a 6TB Green Drive to my DroboPro ( which already had 3 3TB drives ) and while everything seems fine and the Dashboard reports the system health the speed of writing is now terribly slow. 5GB took about an hour.

The Drobo is connected via firewire 800.

I don’t know if this matters but the Usage indicates that only about half the 6TB drive is being used, with the rest reserved for expansion.