To safely shut down your Drobo, open the Drobo Dashboard, click on your Drobo, click the Tools option from the Navigation menu, and then click Shutdown. It may take several minutes to fully shut down. Once it is shut down all lights on the Drobo will be off. For more information a related knowledge base article is available at: https://myproducts.drobo.com/article/AA-00907
It does not matter which slot you insert the drives into, just make sure the drive is securely seated.