Hello,
this is normal behaviour.
WHS will fill the most used disk first, and then jump over to the next. Only shared folders with duplication on will also use a second disk.
WHS will also try to keep the D: drive almost empty (with Power Pack 1 installed), but will not by itself move files around, which are already stored there.
If you want to trigger this, you can try
LZreallocator.
Best greetings from Germany
Olaf