Check out this great utility for your Windows Home Server, Drive Balancer. It’s designed to rebalance the storage across your pool. It came in handy in my case because I added a new 2TB drive (formerly 4x500GB, now 3x500+1x2TB). I kept waiting for the new drive to start getting files, but that never happened. Then I ripped a Blu Ray and the stupid thing started telling me that there wasn’t enough space to duplicate the new file...grrr...just move some files around and there’s plenty of room!! That’s what this utility does, it solves the literally only gripe I’ve found with this otherwise excellent product.
Be sure to read the readme and walkthrough before you get started. Interestingly, it doesn’t move the files, it just adds blank files in a way that, when triggered by the utility, the built-in file mirroring ends up rebalancing. Definitely add it to your toolkit.