The ATOM Hard Disc interface is an internal IDE hard disc interface that fits to the Sam Drive connector.
The interface consists of a 16 to 8 bit bus converter, address decoder and a bus stabiliser to make the rather slow Z80 bus interface with the fast IDE bus.
The interface takes it power from the SAM. A single 2 inch drive which consumes not more than 500 mA can also be powered by SAM. But Other drives must be powered by a separate power supply.
See Edwin’s ATOM Page for full details.
The ATOM is merely the IDE interface, the operating system to drive it is B-DOS.
A popular media to use rather than a full IDE drive is a Compact Flash (CF) card which prompted the development of the Atom-Lite.