Computer is Digital Electronics device but which part is Mechanical?

It depends on the Hardware type which involve mechanical operations,

for example: Hard Disk, CD ROM,... etc

All parts of computer electronic digital only fan Processor and DVD as it contains mechanical parts

If you are referring to the general meaning of mechanical engineering, meaning the whole unit of the machines are based on mechanical engineering in terms of designing and manufacturing and how they have made it its digital electronics. I have read one of the article from the internet and it will tell you in details what mechanical meaning is. Here's the article below, 

Mechanical engineering is a diverse subject that derives its breadth from the need to design and manufacture everything from small individual parts and devices (e.g., microscale sensors and inkjet printer nozzles) to large systems (e.g., spacecraft and machine tools). The role of a mechanical engineer is to take a product from an idea to the marketplace. In order to accomplish this, a broad range of skills are needed. The mechanical engineer needs to acquire particular skills and knowledge. He/she needs to understand the forces and the thermal environment that a product, its parts, or its subsystems will encounter; to design them for functionality, aesthetics, and the ability to withstand the forces and the thermal environment they will be subjected to; and to determine the best way to manufacture them and ensure they will operate without failure. Perhaps the one skill that is the mechanical engineer’s exclusive domain is the ability to analyze and design objects and systems with motion.

Since these skills are required for virtually everything that is made, mechanical engineering is perhaps the broadest and most diverse of engineering disciplines. Mechanical engineers play a central role in such industries as automotive (from the car chassis to its every subsystem—engine, transmission, sensors); aerospace (airplanes, aircraft engines, control systems for airplanes and spacecraft); biotechnology (implants, prosthetic devices, fluidic systems for pharmaceutical industries); computers and electronics (disk drives, printers, cooling systems, semiconductor tools); microelectromechanical systems, or MEMS (sensors, actuators, micropower generation); energy conversion (gas turbines, wind turbines, solar energy, fuel cells); environmental control (HVAC, air-conditioning, refrigeration, compressors); automation (robots, data and image acquisition, recognition, control); manufacturing (machining, machine tools, prototyping, microfabrication).

To put it simply, mechanical engineering deals with anything that moves, including the human body, a very complex machine. Mechanical engineers learn about materials, solid and fluid mechanics, thermodynamics, heat transfer, control, instrumentation, design, and manufacturing to understand mechanical systems. Specialized mechanical engineering subjects include biomechanics, cartilage-tissue engineering, energy conversion, laser-assisted materials processing, combustion, MEMS, microfluidic devices, fracture mechanics, nanomechanics, mechanisms, micropower generation, tribology (friction and wear), and vibrations. The American Society of Mechanical Engineers (ASME) currently lists36 technical divisions, from advanced energy systems and aerospace engineering to solid-waste engineering and textile engineering.

The breadth of the mechanical engineering discipline allows students a variety of career options beyond some of the industries listed above. Regardless of the particular path they envision for themselves after they graduate, their education will have provided them with the creative thinking that allows them to design an exciting product or system, the analytical tools to achieve their design goals, the ability to overcome all constraints, and the teamwork needed to design, market, and produce a system. These valuable skills could also launch a career in medicine, law, consulting, management, banking, finance, and so on.

For those interested in applied scientific and mathematical aspects of the discipline, graduate study in mechanical engineering can lead to a career of research and teaching.

HDD and Optical Drive

A mechanical computer is built from mechanical components such as levers and gears, rather than electronic components. 

Aside from fans and power supply which contain mechanical part, all computer plugs and slots are ussually mechanically designed, because you cannot insert a cable which is not design for that particular slot. For example the female/male,VGA, UTP cables, this is what they call poka yoke or mistake proofing.

mechanical part or mechanical computer is the one who can stand up without direct electrcity power supply !  and every thing u should start it and do it by urself like1950 !

I would just like to add that apart from hardware and software there is a3rd factor called firmware. To update our PC we do a BIOS UPDATE.

Also when the brand new pc-laptop or desktop do not communicate with the old pc we do a firmware update which makes the printer compatible.

i think defining mechanial electric is not improtant point