To save batteries'lifetime, hard disks can be shutdowned. By using hdparm, it's possible to specify the policy that hard drives should follow.
After reading the article It's confirmed: Gutsy is killing your HDD! on Planète Beranger, I had a look to my hard drives and it seems that my laptop has the same issue:
# smartctl -A /dev/sda |grep -E "(Load_Cycle_Count|Power_On)" 9 Power_On_Hours 0x0032 100 100 000 Old_age Always - 500 193 Load_Cycle_Count 0x0032 076 076 000 Old_age Always - 4947598cycles/hour, it seems to be quite a lot ... and can explain why I lost my previous hard disks some weeks ago after only two years of use. I don't think such a value can reduce the power consumption, just think about the peak of power needed each time the disk is turned on ...
As said previously, the way the hard disk behaves can be altered with hdparm using the option -B (with a parameter from 1 to 255, 225 to disablee the power management)
# hdparm -B255 /dev/sda /dev/sda: setting Advanced Power Management level to disabledAfter setting this value, Load_Cycle_Count doesn't grow up anymore.