I'm not sure whether you are doing a custom install or going with the existing partitions, but are you seeing a window similar to this?
You don't have to go with what is suggested. You can highlight (left click drag) and delete any entry in these input boxes. Then specify your own choices from the drop-down lists for each box.
In your case, since you have multiple drives, you would want to select SDA and clear all the entries. Then select SDB and set it up the way you want. Am I in the ball park here?