Can anyone help me get my Mac Pro to recognize two new hard drives?

[JoeBass]

So I just put two new drives in my older Mac Pro because I kept getting warnings about my start up disc being full. I installed the drives and when I go to the system specs, I can see the two new drives showing there, but I'm still getting the "out of space on your start up drive" and the computer doesn't seem to be automatically sending new files to the new hard drives. Is there some way I can set this up so new files just automatically go to the new drives when the primary drive is filling up? Thanks very much in advance for your help!
Joe

 

[Lufbrarunner]

What is the capacity of the startup drive?
How much free space on the startup drive?
You could move some of the data files from the startup drive to one of the installed drives, to free up some space.

 

[JoeBass]

I guess I could, but I thought the point of adding the new drives was that when I needed more space, the data would automatically be sent to the new drives if the startup drive was full. Is there anyway to get the setup to function that way?