MacBook :: MacBook 5,1 Frequently Crashing Hard Drives

I have a MacBook model 5,1 (uni-body late 2008) that I had to replace its hard disk like 3 times in the last 1,5 year (maybe more).

Please note that the laptop is used in a flat, stale table, along with other laptop and an external hard drive (turned on most of the day and night). Also, I don't carry my MacBook everywhere with me. It stands on the table most of the time, like 90% of the time actually. One thing to note though is, I keep the MB turned on for long periods (~15 days), not doing very frequent reboots.

The symptom is always the same: it all starts with general sluggish performance and then, beach ball every once in a while. When I go check the system logs (/private/var/log/system.log) I find a couple of entries like: kernel[0]: disk1s3: I/O error. It's inevitable the hard drive exchange a couple of days later.

So my question would be: could it be an issue with the HD controller? The sudden sense motion component? Or what else?