The big drawbacks of this product over a Drobo Pro are that it holds two less disks (6 versus 8) and that it can only protect for up to 2 disk failures while the Drobo protects up to 2 SIMULTANEOUS disk failures. Let me explain the difference... In the ReadyNAS, you can have up to two disks fail at any time if so configured. But you must replace those disks in order to gain back any protection at all. In other words, those disks fail but no rebuild is performed until you put in new disks. With the Drobo, the system will start rebuilding each time a disk fails. That means once the rebuild is complete you can have another two disks fail, provided you have enough free space of course. The unit not only dynamically expands, but it can dynamically contract if you have little enough data to do so.