I had the same problem. In order for DPM to use the full amount of disk you must delete any volumes using Disk Manager. In order to use all of the disk have it formatted as a Dynamic Disk with Unallocated Space. Do not create a Volume and assign a Drive Letter!!! Then use the DPM Disk Management to Add those disks. This time they will show the full amount of the disk as unallocated space. You can then assign that space to a storage Pool. Don't worry, took me an entire day to figure this out.
Look here for additional guidance.
http://www.windowsnetworking.com/articles_tutorials/Deploying-Data-Protection-Manager-2007-Part2.htmlJie-Feng Ren / Torsten Your posts were as helpful as a two headed Mule at a chicken catching contest! They would have been helpful if you posted a link to an acutal DPM forum of which I have yet to find one. I found this post because I was searching for an answer to the same problem.