Comment 5 for bug 875719

I just installed hdparm v9.37 from source. I set the S option to 1 (5 seconds) and it worked. But 20 seconds later the disk spun up again (I did not access it). So I tried the Ubuntu version of hdparm with -S set to 1---and it worked, too. But the disk spun up again after 20 seconds. I guess Ubuntu's hdparm is not broken but some process accessing the disk prevents the disk to spin down which only occurs with -S set up high enough. How can I detect which process prevents the disk from spinning down?