Unlike most DIY hard drive clocks which simply use the platters as a backdrop for a standard clock mechanism, the “Harddisc-Uhr” (dig that fancy font!) actually rotates the platter itself to change the hour, while using an arrow attached to the read/write head to indicate the minute.
Project Page (German) [Heise.de via Crunchgear via Hackaday]
Wow, dig that 5-1/4″ platter and linear drive for the head gimbal assembly. Might be a little harder to do this with a present day voice coil actuated device.