One other point worth considering - the file sharing software. We've a 12TB NAS attached to our cluster via an Infiband switch. We're using GlusterFS as the file sharing software. It appears to be quite scalable both for adding new nodes and for adding more storage space. We're working on the principle of fast disks for short term immediately needed data, slower disks for mid term less needed but still wanted data and slower disks or tape for less needed, data, long term storage. Gluster allows us to manage this set up.