Q. I've added new disks to my Storage Pool. How do I rebalance the data across the new disks?
A. When data is written to a virtual disk that is created in a Storage Pool that data is spread over the disks based on a number of columns (the number of disks used to write a stripe of data) and the interleave (the amount of data written to each disk as part of a stripe). If you have written a large amount of data and then add additional disks to the Storage Pool then new data written will start writing to new disks (in addition to existing) but existing content will not be moved to the new disks to rebalance data equally across disks.
If you wish to perform this type of rebalancing the only solution in/2012 R2 is to copy the data to the disk which will then distribute it evenly across the disks then delete the original data.
Windows Server 2016 and Windows 10 has a solution in the form of the Optimize-StoragePool cmdlet which will rebalance data across disks which means if you add new disks to a Storage Pool run the command:
Optimize-StoragePool -FriendlyName "Storage Pool Name"
which will perform the rebalancing. More information can be found at https://technet.microsoft.com/en-us/library/mt126109.aspx in the Storage Spaces Optimize Pool section.