We have been asked many times "Can we have a 3TB Primary HDD?"

The answer is no!  There are many technical reasons MBR/GPT/Bootable etc that stop Tranquil from supplying 3TB HDDs as primary drives, but there is also a hidden concern, even if the bootable issues can be resolved.  

Assuming you could boot from a 3TB HDD (GPT mode) and then you added the 750GB (2nd partition) to your storage pool - and then enabled folder duplication (assuming you were using MonStore), the problem is that with more HDDs connected, it's possible that the 'duplicated folder / files' you are protecting could be on BOTH the primary and the secondary partition of the 3TB HDD.  This is not ideal, as loosing the HDD may loose both folders.

In the same way, using WHS2011 Backup to the second partition would be unwise, it's on the same disk as the original data!