Sounds to me like Background Media Scan, sure they would prefer to say 'defrag' over at support, point is a HD on its own can't defrag anything, it only looks at sector level and what it does at such high density drives is to constantly check in sequence that each sector can actually be read and if suspect move its data around to a more healthy sector.
The bad thing, the way I see it of course, is that this is done continuously over the lifetime of the HD and the HD is designed around this constant data maintenance 'feature', to me that says that the manufacturer does not trust its own media to hold the data reliably at such recording densities from day one, let alone over the projected lifetime of such a product.
Just look it up, everyone is doing it from some capacity upwards or from some recording density upwards. No-one is advertising it though and surely it goes undetected in a noisy server environment.