They use regular consumer grade HDDs on the cheaper servers, and enterprise grade on the slightly more expensive ones. For instance, their EX6 servers have 2x 3TB of high quality hard drives (€70/month) and their EX5 servers have 2x1.5TB consumer grade hard drives (€56/month).

For comparison see: http://www.hetzner.de/en/hosting/produktmatrix/rootserver-pr...