thanks for the idea man...... 
i've thought of this b4.. but afraid if i might use it..
can i just use usb floppy for laptop ??

Harddisk - Heatsink mod
What you need:
1. A hard disk (duh!)
2. Few aluminum heatsinks (duh duh!). Aluminum dissipates heat better.
3. Thermal paste.
I used the paste I got with SI-120.
4. Isopropyl alcohol.
I used spirit used by doctors. May not be as good as IPA but does
the job and is better than using after-shaves/nail polish removers.
5. Cotton swabs (without your earwax ofcourse)
6. Hammer. So you can smash your head if your HDD crashes.

What you get:
A cooler hard disk which will be grateful to you for implementing this mod.

I have 3 HDDs in my cabinet and inspite of managing the cable to increase the airflow, the HDD temps always reached 47-48 deg C. Sometimes even 50. Too hot for me. Connecting the HDDs in front of the 120mm front intake didn't help either as the temps only dipped about a couple of degrees.

During one of my extensive weekend cleaning sessions, I noticed the smooth metal surface on the HDDs (Hitachis in particular as the label is very small and stuck at the bottom, leaving ample clean, smooth metal surface for our use) and thought why not attach a few heatsinks to try and dissipate the heat. You can remove the labels if you don't have enough space to attach the heatsinks but make sure you know what you are doing. Removing the lables may void your warranty.

1. Take a clean cotton swab, dip it in IPA (spirit) and clean the
whole metal surface of the HDD. Use the other dry end to soak
up the remaining fluid. Do the same for the heatsink bottom.

2. Squeeze a tiny amount of the thermal paste on the bottom of the
heatsink and press (and I mean gently. Don't press it like you are
fixing postage stamps.) it on to the metal surface of the HDD. Twist
the heatsink in both directions to remove any air bubbles and to
spread the thermal paste.

3. Do this for all other heatsinks.

4. I've arranged the heatsinks at an angle as this should create some
turbulence and increase the heatsink surface area coming in contact
with air. If you fix the heatsinks in a straight line, air will just flow past
the heatsinks without coming much in contact with the HS surfaces,
thus reducing the heat dessipation.

5. Repeat the above steps for all HDDs.

Source:LINK

topic starter. your design either circulates the air in and out and in and out or its not efficient in cooling. it depends on how you want to design the cooling actually. normally if you want to cool the hot hard disk. other than wc, passive cooling is too hard to cool it up. frankly if you have alot of hard disk. try placing them close together and fan them directly. any fan will do, but do remind it has to have a high preassure and prefereable high cfm to maximise cooling. my hard disk mod is the same as your latest one. but mine is a large 12cm nidec beta V at one side while another delta in front of the casing. exhausting hot air is not a concern to me as my casing is naked. for the case of dust. i think there is not much you can do about it. if you're in an air con room then its good as the air con will filter most of the dust. but if you're on the plain ceiling fan, i suggest you install a air filter. its cheap and keeps you from doing daily house keeping on the pc. my friend is currently trying out another method of reducing dust without the usage of filter. he's thinking on using static energy but its kinda dangerous as  electrical component might get faulty due to it.

if your place your hardware too far apart and cooled on a parallel air way. then its not good. since you have plenty of space tryblowing it. if dust is your concern, you can try salvaging old heat sink and then blow or suck on the heat sink attached to the hard disk. work fine for me.
always remember hot air rises. cold air is always at the bottom. to force air thru tight places, high pressure fans are required( fast spinning fans) high cfms fan work best in moderating and removing heat fast.