Using Vail build 7600 and was able to replicate problem on Windows XP, 7 and Snow leopard client. Simple 2 drive configuration (160 GB as system) and 2 x 1TB drives. 1 drive in storage pool so no duplication and one drive outside of pool formatted as NTFS.
I reported as bug on the connect website and hoping for a quicker response or others who may have encountered this.
The wrong free space is reported when mapping to the default shares or shares created within the storage pool aka using the WHS console. The reported free space is 1.27GB and total size is correct (1.2TB). I created a manual share on the non-pool drive and
it's reporting the correct free space.