(I'm going to pin the thread, as it is commonly asked)
StableBit DrivePool v1.X is meant for WHS2011 (and that line of OS's). It features tight integration into the dashboard, and most of the same features.
StableBit DrivePool v2.X introduces it's own remote control scheme, supports reparse points (if you're not sure what this is, then you don't need it), and most importantly: File placement rules (allows you to specify which files or folders end up on which disk in the pool).
The license is valid for both products, and you can switch between them, if you are inclined.
Additionally, the pool is backwards and forwards compatible (for the most part).
We recommend the 1.X version unless you need one of the listed features. Or you can try both and figure out which you like more.
Microsoft MVP for Windows Home Server 2009-2012
Lead Moderator for We Got Served
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This is my server
Lots of "Other" data on your pool? Read about what it is here.