What do to with those old disk drives...

[Human No More]

It's a slot cut in the platter, and the LEDs are underneath (you can see light leaking out from the edges of the platter). The slot allows light out from LEDs from a single section at a time, allowing the colours to be combined so the LEDs can create RGB colours, and the slot prevents them from interfering with each other. The image is actually formed completely beneath the disk from the LEDs, the slot just filters them. Similar displays where the LEDs move can only use a single colour.