no way it could only run on ram because there wouldn't be enough storage space,and it would be deleted every time you turn off you pc.
cant be only hard drive cause the transfer rates are too slow
cant be only cache cause that is like 32kb in hdd's and maybe a couple megabytes in a processor, and its not user accessible.
Closest thing would be a solid state drive. They are bleeding edge technology but are available for home computers.
http://www.tigerdirect.ca/applications/SearchTools/item-details.asp?EdpNo=4350446&CatId=4217