improvements on PureDisk type

hello. i'm in the middle of replacing my PureDisk type storage and found out that one cannot add nor remove hard disks to the existing PureDisk storage.

it would be nice, and flexible, in future releases of NBU to allow adding/removing of hard disks of type PureDisk.

note: i'm not talking about expanding current hard disks. i'm referring to addition and/or removal of hard disks that are type PureDisk.

hth.