[PROBLEM] HDD wakes up himself after some time

[feg]

Hi,

I have issue with my external hdd (it's WD Elements Desktop connected directly to my raspberryPi - it's 3,5
" drive, so has own power supply).
HDD is waking up after a few minutes after sleep (sleep is after 30 minutes of inactivity). This problem is only on XBMC distributions - on Xbian, but I also tried Raspbmc.

On Windows and Raspbian HDD works properly and doesn't wake up himself.

I'm pretty sure that nothing uses drive and he nonetheless wakes up.

Anyone can help me discover what wakes this drive?