Chris Gray, an MS development lead on WHS, has posted a fantastic (and short) tutorial on the logic behind the WHS Auto-migration, duplication and balancing. I recommend this as reading for all as it will help you understand what physical drives ("volumes") your data may be stored on (and why).

There is more to Data Extender than I thought!

Here's the link:
http://blogs.msdn.com/chrisgray/archive/2008/07/04/a-brief-description-of-the-balancing-algorithms-used-in-power-pack-1-s-drive-extender.aspx

Note: I immediately wanted to edit my registry to tweek the settings, only to find that the registry entries are not present in WHS PP1 Beta (1771). Not sure why, but you will see my comment/query in his blog. I also, in another comment, asked him to comment about external storage, USB, eSATA and eSATA Arrays.

By the way, he has some other interesting entries in there as well!

PS: Thank-you "GeorgeL" (who/where-ever you are) for e-mailing me this link.