[PROBLEM] HDD wakes up himself after some time

[Alcoinus]

You tried the tool? Maybe it works.
And the problem is that the system can't read the power state of the HDD when it is in standby mode. I checked this with hdparm. When my HDD was on standby I did hdparm -C /dev/sda and the HDD woke up. But when it is only on spin down the command returns correct the power state.

So you can't say whether the issue is because of xbian or because of the way of WD standby works.