Which component comes in two varieties—one that mechanically writes binary data onto spinning magnetic disks and one that writes data with no mechanical parts?

The motherboard is a central circuit board that connects all components of a computer, including the CPU, RAM, and storage devices. It does not have different varieties based on data writing mechanisms, as its primary function is to facilitate communication between various hardware components rather than to store data itself.

The system clock is responsible for synchronizing the operations of a computer's components, ensuring that processes occur in a coordinated manner. It does not relate to data storage or writing, nor does it have mechanical and non-mechanical varieties, making it irrelevant to the question.

An expansion bus is a communication system that transfers data between various components of a computer, allowing the addition of expansion cards like graphics or sound cards. It does not pertain to the writing of binary data and lacks the duality of mechanical and non-mechanical types described in the question.