If you're familiar with DPM 2006, adding disks to the storage pool in DPM 2007 might be confusing because it doesn't behave the way you expect. You're not alone - a lot of you using the beta 2 version of DPM 2007 have run into this. So here's an explanation of DPM 2007 and the storage pool disks.
In DPM 2006, disks added to the DPM storage pool are reformatted and, if necessary, converted to dynamic disks. DPM 2006 uses all space on the disk in the storage pool.
In DPM 2007, DPM uses only unallocated space on disks added to the storage pool. If you add a disk to the DPM 2007 storage pool that has existing volumes on it, DPM cannot use the space in the existing volumes. To make an entire disk available to the storage pool, delete all existing volumes from the disk before you add it to the storage pool.