[PROBLEM] HDD wakeup from idle constantly

[Aeefire]

Hey guys,

I am using my raspberry pi, like probably most of you, with an external hdd ( http://www.amazon.de/gp/product/B007AG1V1Y/ref=oh_details_o09_s00_i00?ie=UTF8&psc=1 ). It's formatted to ext4 and generally works great.

My problem is though, my raspberrypi is stationed in my sleeping room, because my TV is there. However, when the Pi is not being used (e.g. xbmc is not being used), the HDD goes to idle (sleeps). This is generally good, as it's relatively loud. THe problem starts, when the HDD somehow wakes up during the night (probably also during the day, but I just notice it when sleeping) without me using the Pi.
I notice it multiple times a night. It wakes up and immediately seems to go to sleep again when it's finished waking up.

Is there any reason for this? Is this rpi/xbian specific or caused by the hdd? Is there a way to avoid that behaviour?