If you have any data on your drives, copy the data off.
Drobo will erase any existing data.
To see if you are using the right drives for your environment, check: http://www.drobo.com/storage-products/choose-drive/drives
It does not matter which slot you insert the drives into, just make sure the drive is securely seated.
The Drobo should be powered off during this process.
Is the Drobo operating normally?
Are all the Drive Lights Green?
Is the data acessisble on the Drobo?
Have you upgraded to the lastest firmware?