There is a useful project at this site that I duplicated. It's a switch that selects between 2 SATA HDDs (it switches power between two SATA power connectors so that only 1 is active).
It's great for testing alternative operating systems or having a quick-boot surfing system on one hdd and a work related system on the other for example.
The build process is pretty much outlined on the site. Here's how mine looks using a big old 2 deck rotary switch
I had to saw quite a bit off the shaft and spraypaint the knob for it look passable. This is how it looked beforehand: