When you have disks that are physically attached to a server that is running Microsoft System Center 2012 Data Protection Manager (DPM 2012 or DPM 2012 R2), the disks are displayed in Disk Management as expected, however when you try to add them to the Data Protection Manager disk pool, Data Protection Manager may not enumerate some of the disks. Because of this, they cannot be added to the disk pool. No error is generated in the GUI and a rescan does not help. Also, when you use the
command in the Data Protection Manager Management Shell, these disks are not enumerated there as well.
For all the details regarding this issue, why it occurs and how to resolve it, please see the following: