As of three days ago, I had two 500 Gigabyte drives and 1 Terabyte drive installed in the Drobo. Two days ago a flashing red light appeared beside one of the 500 Gig drives. This meant that the drive had failed. I purchased a 1 Terabyte Western Digital replacement drive for $114 at infonec.
True to data robotics claims, I was able to hot swap out the defective 500 Gigabyte drive and slide in the new Terabyte drive without incident. It took about 15 seconds to do. Subsequently, it took about six hours for Drobo to reconstitute data redundancy – ie: to format the new drive and redistribute my data across the newly constituted drive array such that data would once again not be lost if any drive failed.