Amazon has 1 TB drives for $99.95 both LaCie (I have two of these connected to my computer and have had no problems over a number of years) and Iomega.
For sure, 622/722 do accept 1.5 TB.
211 is not [yet].
If you'll test it in your 612 and tell us here, it would benefit for all members.
[Get it in that store who will give full refund in case of failure - Fry's did that for my 2 TB tests].
Not platters, but disks - separate devices inside of the enclosure. Number of platters inside of each disk is irrelevant for EHD PURPOSE ! Two posts above explained it for you.
Ask manufacturer or read tech specs of the device.