This is my expandable K'nex Hard Drive Rack, with pictures showing a step-by-step creation of a detachable triple 120 mm fan cooling mod powered by an old hard drive shell:

I ultimately started to worry a bit about the heat being generated by all those hard drives. They are crammed together, but still have a fair bit of breathing room. Nevertheless, I came across a good deal on 3 used 120 mm fans, so the next morning I set to work making a detachable cooling mod for it.

I thought for a while about an easy way to power the fans, making them swappable if I chose to do so later (aka not damaging the molex plugs on the fans). I finally came up with a great idea the next morning. I had a spare hard drive shell that constantly fails and doesn't really work from a friend's old hard drive, so I stripped it down to just the circuit board and the molex power plug coming off of it. I hooked the molex adapter up to a daisy chain of the 3 fan molex adapters to give them power.

All the parts I used in the construction of the cooling mod:

Starting construction a base to hold the fans:

Fans mounted in place, tied off using Cat5 wiring:

The cooling mod mounted on the front of my hard drive rack (set to blow backwards onto the hard drives to keep them cool):

Wired in an additional switch to the top of the fan mod structure to turn it on and off - it controls the break in the +12v (molex yellow) wire that I made:

And now to connect all the wiring together.

The circuit board is actually the barebone guts from a hard drive shell, as previously stated. I will be utilizing the molex plug that usually powers the hard drives to power the 3 molex powered fans instead.

I made a break in the +12v (molex yellow) wire, which you can see in the upper-right hand corner of the picture - the break has a 9v clip attached to it, which will connect to the 9v clip connected to the grey wire (top middle). The grey wire is connected to the fan - the 9v clips allow for the mod to be easily detached if needed.

I coated all of the wire connections with hot glue after soldering them to prevent unwanted electrical current going to places it shouldn't. The circuit board and wiring is secured in place using Cat5 wiring and hot glue as well.

The final product, all finished up and plugged in:

A closeup of all the molex plugs and wiring crammed together above the hard drive shell circuit: