Skottie> some in-band solutions:
Skottie> Acopia (www.acopianetworks.com) Skottie> Rainfinity (rainfinity.com)
I'm not sure I could call these two a real HSM system, though they do what the original poster wanted, which is to migrate data from expensive disks to less expensive disks. But, we've been burned by Rainfinity in the past, and we're thining about Acopia now. Acopia really looks good. But money and some limitations it has currently make us hesitant. Of course I'm not speaking for my company in any official way here.
But I can say that Acopia has a nice setup, and the newest version of their OS should remove and change some restrictions which make it very tempting to deploy.
I guess I think of both of these systems as more virtualization systems. Sorta like the VxVM of NFS. You mount the NFS volumes the Acopia exports, and it shuffles that data around behind on the storage without your clients noticing or caring. Just like VxVM.
Skottie> NeoPath (www.neopathnetworks.com)
Haven't looked at these folks, any good/bad thoughts?
John