Hard drive swap but no rebuild?

I wanted to upgrade a drive in my 5n2, so I took out a 160gb drive and when I did it started it’s rebuild (common). Once it was completed, I put in a new 2tb drive and the bay light immediately went solid green. There was no rebuild, all lights are now solid green and Dashboard recognizes the new drive.
Is this normal (for no rebuild)? The 2 tb drive came out of another Drobo (which I upgraded that one from 2tb to 4tb).