If the DroboPro volume has been added to the storage pool WHS will automatically use it for storage of client backup (once D drive starts running out of space). Please beware that the client backups may get spread over multiple physical disks,
so failure of a single disk will mean loss of all backups. If you really need to keep your backups you can use the
WHS BDBB Add-in to copy the backups to another disk. When your server has only a single disk and the DroboPro has been added to the storage pool you can enable duplication (option in
the WHS BDBB Add-in) for the backup database to ensure you always have a full copy of the backup database on the DroboPro
There is no way you can force WHS to store backups on a disk that has not been added to the storage pool.
Henk Panneman - http://www.homeserverweb.nl