16.04 keeps waking up unmounted USB drives

I don't know if anybody else has experienced this, but i did on two systems on which i installed 16.04.

Essentially, if i plug a USB drive, unmount it (eject it in Nautilus), and put it to sleep (hdparm -y), i can hear the motor spinning down.

However, after a variable amount of time, the disk spins up again, and hdparm -C reports the disk to be active/idle again.

I also tried to disable gvfs from handling the drive, by putting its uuid into fstab, but even this does not help. Even with the disk unmounted, i keep getting it woken up by the system and i don't know why.

Can somebody suggest a way to diagnose what process might be responsible?