One of the reasons I register my hardware is to get notifications from the companies for exactly something like this! Data Robotics sent out an email to current Drobo owners... come and get a second one at a nifty discount. It's a great deal too. So, if you haven't checked your email or you overlooked that little tidbit from Data Robotics, go back and look at the offer. It might just be worth it to you to get an additional unit.
If you haven't gotten off the fence yet and purchased a Drobo, you can still get a discount. The $25 evangelist discount is good through December 31st. Just use the REFJENL code for the instant discount.
If you have questions you need answered before making the purchase decision, check out the Drobo Community and get the skinny on how folks are making use of their units.