www.minecraftforum.net/topic/1521315-most-complex-piston-creation-in-mcxbla-irpe-11/
^^^ Check out my MCF thread about this machine, with more screenshots and details!

This is my redstone interpretation of an HDD. It holds eight 29-bit piston tapes that contain read-only data. The "discs" themselves can easily run on a 5-tick clock cycle. But, thanks to the counter I'm using they run on a 7-tick clock, which is still a decent speed.

The indexing mechanism is a modified 5-bit binary counter, which resets itself when it reaches 11101, or 29. The counter resets instantly, and because of this it is impossible to seek line 29. When attempted, the counter is stuck in an infinite loop until the clock is stopped or the destination is changed. Reaching the original default location, line 0 is also impossible because of a design error on my part, which I didn't notice until I was almost done. (herp derp)

The seeking mechanism wasn't planned out very well and is also a bit buggy. The counter's outputs are inverted, and when pistons are selectively activated by levers, the signals are sent to a line which inverts and powers a torch in the clock. Therefore, whatever digits you select, their inverted outputs must all be off to shut down the clock. This works fine when seeking from the default location, but has a lot of issues when attempting to loop back to a previous line.

This mechanism still has quite a few bugs, which is why I don't consider it complete yet. I may work them out sometime soon, or just wait until I start on a completely new design. Not completely sure at the moment.

Additional Notes

Created in the XBLA version of Minecraft.