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://drobo.staging.wpengine.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.