Hi,
If you have multiple drives, the D: partion, apart from being used in the staging process when loading files, won't have any data on it. When your other drives fill up, apart from about 20GB, which WHS reserves, then data will be stored on the D: drive as a last resort.
However, if you already have data on the D: drive, for example, you only have two drives, and then you add further drives, that data will remain on the D: drive while new data is loaded on the new drives. If you go into the shares and maybe move or alter the files, (with duplication turned Off,) then re-enable duplication, WHS will probably then move what it now thinks are news files, to the new drives. - Does that make sense?
If it doesn't, you might need to move the files off the shares and then back on, to trigger the redistribution.
Colin