From a constructivistic perspective on learning, it is essential to build on learners' previous knowledge in learning scenarios. Since computing system hardware design and principles of computing system organization constitute key parts of computer science classes, the identification of respective pre-conceptions is one of the core areas of computer science education. According to cognitive science learning aspects, the knowledge of the individual parts of which such systems are composed plays a decisive role in understanding them. Thus, this paper presents a self-contained substudy of secondary school students' mental modelling of computing systems with special focus on what we conceptualize as part-whole-thinking. Sixty eight secondary school students were asked to draw their conceptions of what three exemplary computing systems (smartphones, video gaming consoles, and robotic vacuum cleaners) look like from the inside. A content analysis of the 204 drawings received was carried out from evaluative, quantitative, and qualitative points of view. From the mental models expressed by the students in their drawings, insights into common conceptions are derived. Looking ahead, recommendations can be implicated from the main results.