Storage servers sit under the control of a media server in the architecture hierarchy and just like how a master server can also be a media server, a storage server can also be a master or media server. It's a nicely scalable solution since all these tiers can be on one host but as you scale up and out you have the option to start breaking out dedicated media servers and storage servers to load balance or provide multiple points of redundancy (this is backup after all).
Where storage servers break entirely new ground though is in the concept that they don't need to be a NetBackup server at all. This has never been done before with NBU since masters and media servers have always had a software component you install before they became a master or media. By opening up storage servers to 3rd parties, OpenStorage was born with an API to allow NBU to talk with these servers.
The basic reasoning behind having a storage server as a new 4th tier has to do with the fact that storage servers purpose in life is to manage disk. And unlike tape, dealing with disk comes with a lot of things like volume managers and file systems that may not work across different operating systems that media servers are running on. Media servers send data in a generic format and it wouldn't be good if it was stored in a proprietary format that only that media server or operating system could read. Think about a media server writing to a Windows NTFS file system and your Solaris media servers can't access that data anymore. NetBackup is used in a lot of large heterogeneous IT shops so not breaking the media server concept was important. Storage servers abstract the way disk is managed so media servers can continue to write a generic data format and the storage server can figure out how to store the actual data in a way it can be recalled by any other media server regardless of its platform.
For AdvancedDisk the storage server is the media server itself which means the filesystems do have to match across all the media/storage servers accessing those disks. This is the entry level disk option that lets you use all the storage server benefits of load balancing and redundancy but it comes with the limitation that all the media servers have to match in terms of the operating systems.
For OpenStorage, PureDisk and the NetBackup 5000 the storage server is outside the media server. The storage server is the DataDomain controller/head, the PureDisk Content Router or the NetBackup 5000 server. Most all of these storage servers are running a custom build of Linux and therefore handle the disks either using native Linux filesystems (EXT/riser) or in the case of Symantec we like to use Storage Foundation (it is after all our own kick-ass file system and volume manager). NetBackup media servers just send data to the storage server and it deals with all that disk stuff.
So hopefully that clears up the concept of a storage server. It's not very often that at whole new piece of architecture is added to NBU. Storage servers have legs so you'll definitely see it to be extended even further as the concept for how NetBackup manages disk.
