My colleague and I are having a debate and would appreciate your opinion. The question is: How much of the power consumed by a disk drive is thermally dissipated to the surroundings? My answer is very nearly 100%. The only energy that isn't thermally dissipated is a small amount of electrical energy of the interface signals plus whatever vibration energy is absorbed by the mounting structure. I came to this conclusion by considering a control volume surrounding the disk drive. Inputs are electrical power and outputs are electrical signals, mechanical vibration, and thermal energy. My colleague insists that a substantial amount of the energy is used to spin the disks and move the actuator, the rest being thermal. I think the motion of the internal mechanical components is irrelevant except the extent to which it contributes to vibration that is absorbed by the surrounding structure.