Understand this sequence will help you in future to determine why and how:
When you first create a disk storage unit (using AdvancedDisk, PureDisk, DataDomain etc.), you will need to:
1) Create a storage server
2) Create a disk pool
3) Create a disk volume (which maybe automatically taken care of by Netbackup)
4) Create a disk storage unit
5) Start backup (writing images into disk)
So when you go the other way round, to delete a disk pool, you will need to first:
5) Remove images from the disk
4) Delete the disk storage unit
3) Delete the disk volume
...before you can delete the disk pool.
Another important note is, before deleting disk volume and disk pool, you need to make sure you set their state to DOWN. That can be done using nbdevconfig -changestate command, you can look for more details of that command using the Netbackup CLI admin guide.
