I suppose if you have the $$$ some Solid state HDs could be the quietest available.
I use a 1TB Iomega NAS. I also use a WD 'My Book' USB, and have a couple USB & SATA enclosures so I can exchange OS drives and back them up from one pc to another.
The computer audiophile dot com offers a few options for you. The post alluding to a wireless configuration which allows you to sequester your server/drive elsewhere is a good one.
I found if you use compliant nodes or pods beneath your ??? the noise from that drive or server will drop drastically. A bit more if you also have it connected to a good cond. AND the sound improves too.... but don't take my word for it, try it for yourself.
The Iomega X NAS has two USB ports so it can be added to. Under $300 as well. great reachable support too!
I've got a Seagate 1TB Barracuda and a few WD drives with no issues so far.
I am going to go with a hot swap-able server of sorts soon as I can though.
Good luck... try those Bright Star nodes or Vibra pods... they provide a more than noticeable lessening of the SPL them gizmos produce!