The reuse of clinical health data holds immense promise for advancing medical research, yet remains constrained by complex legal, technical, and organisational barriers. This article examines these challenges through the case study of TumorScope, a Belgian interdisciplinary initiative developing a secure, multimodal data environment for glioblastoma research. Drawing on five years of practical experience integrating imaging, genetic, tissue-based, and clinical datasets, the study identifies key legal, ethical, technical, and operational obstacles to effective data access, linkage, and reuse. Technical issues included fragmented data flows, pseudonymisation complexities, and limited interoperability, while legal and ethical barriers arose from strict interpretations of the General Data Protection Regulation, medical secrecy obligations, and intellectual property constraints. These were compounded by operational challenges such as unclear governance structures, resource limitations, and the limited capacity of Medical Research Ethics Committees to assess data-driven research. The analysis further considers the European Health Data Space Regulation (EHDS) as a potential enabler of responsible secondary data use, while noting uncertainties in its national implementation. Overall, the study demonstrates that meaningful health data reuse requires more than regulatory compliance, it depends on robust governance frameworks, institutional coordination, and sustained investment in infrastructure and expertise. The findings contribute to ongoing debates in healthcare informatics on how to translate the vision of the EHDS into practical, ethically grounded data reuse for patient benefit.