hdparm is, somehow, a powerful tools to control harddisks.
It can even force your hd to go sleeping.
for me, I lost a harddisk 2 years ago. The harddisk went sleeping with the order of hdparm, and didn't wake up anymore. I thought there's something wrong with the power supply of motor.
Anyway, it's a good tool and I will utilize it in the future, except of that trying the sleeping performance.