A hard disk problem?

Okay, this is a bit weird so bare with me, I'll give you any information you need just tell me what to do.
I'm a computer technician with some experience but I never had this problem before.
Let's say I am not working on the computer but sitting next to it and watching tv, suddenly I hear my (I assume) hard disk stops working (as if the power cable of it was taken out while it was working). Not too much noise but it can be heard if listened carefully. Then, when I try to access something on the computer like the windows calculator it will not come up fast like it usually should it will almost freeze my computer by thinking\processing and you can hear the hard disk starts to work again (just like you hear it when you turn on the computer from being shut). Some things doesn't trigger that though but until I reach the things that do trigger the hard disk to work again it will remain in a "sleep" mode.
I can still access the hard disk from the computer and open it with it "sleeps" but of course, entering it while it "sleeps" will cause my computer to nearly freeze and start the hard disk which is annoying at times.

Now, I have 2 other hard disks inside that computer which are much older than the one that (I assume) causes the problem. But they are working fine.

I ran a bootable software which is called "MHDD" to test all my hard disks one by one and they all were just fine. No warnings, no errors what so ever.

My system includes:
PSU: 650W Seasonic
Graphic Card: Radeon HD4870
Motherboard: Intel BLKDP55WB Whitesburg
Processor: Intel Core i5-750
Memory: 2x G.Skill DDR3 2048GB 1333Mhz CL 9-9-9-24 NT and 2x 4GB Kingston
Hard disk: Two of Western Digital 250gb and the one I suspect to be the problem, 1.5TB of Seasonic
 
Check the Power Saving features in the Control Panel.

Normally, we kick in the screen saver ~10 minutes
Then hibernate the system after X - - depending upon if you access video/music on that system from elsewhere,
eg another system on the LAN or via your HDTV.
 
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