Just to clarify though. If your master is also a media server (as it usually is) you don't have to have a separate media server to talk to the non-NBU appliances.
In our environment we have the master/media and multiple other media servers that all use storage units defined using NFS mounts to the various appliances we've used (DD, DXi and ExaGrid). That is to say we can access each appliance from all media servers rather than requiring a separate media server for each.
We don't use OST here because of the additional NBU license required so it may be different in an OST setup.