Try this it worked for one user:
"With the 5D running with no disk in it, looking at it in Drobo Dashboard, I pushed my five disks in slowly, one at a time (counting three seconds in between). This feels very risky, but this time the Drobo started seeing them one at a time, gave me the notice "Too many drives have been removed. Put back the drive".
Then it changed from all five red, to started green and yellow blinking. This for two or three minutes. Meanwhile I am madly (and successfully) copying off my urgent data to my local HD. Then it switched to top four green, bottom one red (but there was a drive in it), then finally switched back to all five flashing green and yellow, and gave me the notification "Drobo data protection is progress, please do not remove any drives".
Its been running now for over an hour, still green and yellow flashing, still able to access my data.
I think that putting the partial set of disks in forced the Drobo into data protection mode, and it is rewriting whatever was amiss."