Aiyappa,  I have a hybrid Unity in my lab.  This is an interesting question so I deleted my pools and tried recreating them from scratch.  If I create a new pool or expand a pool, I can confirm the behavior is it selects the non-system drives first.   After selecting the tiers, when you go to the step to select disks, it doesn't let you specify exact disks, but you can tell when it's adding system drives based on the "Usable Capacity." In the example below, if I select 5 of 11 disks, 2.1TB, that doesn't include system drives.  Selecting 10 of 11, 3.8TB includes system drives.  The system drives in that case are only providing 1.7TB of usable capacity, vs the 3.8 when using the non-system drives.  I tried this a few different ways, and bottom line, system drives always go last.  It would be nice if more information was provided at the point you are adding system drives, but I think that may go against the style goal of simplicity with this product.  [IMAGE]
Is there a way we can manually pick drives to be part of a Storage group ? in other words ,  how do we avoid Systems drives becoming part of a storage pool ? 
Unity Drive Selection