FYI, the recycling filter(s) in the drive are there to catch particles coming off the drive platter, not to catch dust particles coming into the drive (as the drive is sealed).  That's my understanding anyway.
 
Checkout "The Engineer Guy"'s video on it. He's pretty cool:
 
http://www.engineerguy.co...os/video-harddrive.htm

thanks to bitflipper for the details, including the high availability feature. For consumer drives, I've actually seen the description being almost "the opposite". Here a NAS drive is intended for one of the consumer NAS devices where the main feature is power save, and ability to get in and out of sleep relatively quickly as the expectation is that the NAS device is for off-line storage and thus not used often. Fascinating!