Big page of links:http://www.rollette.com/railgun/compulsator/L
I don't think it's straightforward to build a compulsator out of a hard drive. I think the hard drive platters would be more suited to a self-excited homopolar generator. I've never built either, but I'm closer to the latter. Use thick copper wire to ensure little resistive loss. There would be a coil (positioned with each turn parallel to the platter, so the field goes through the face of the disk) attached to a brush that runs on the outside of the platter, then another brush on or near the axle of the platter, then another coil on the other side.Put a large current through the thing and it will run as a motor, storing up rotational energy. Then suddenly switch to a load and the magnetic field will collapse and brake the disk, dumping rotational and magnetic energy at the same time. If you spin up the platters by a different torque source (say run as a turbine) then you must provide some "seed" magnetic field to bootstrap the process. Keep the coils shorted until it is time to drive the load.