Can't get the harddrive to spin down

Hi

I have installed Debian testing unto a Cubox Pro with the arm (armhf) architecture and I am running with kernel 3.12.8.

Debian is installed on an external harddrive connected via the upper USB to the box allowing it to boot of the box.

I have tried getting the disk to spin down when not used using sdparm, but some unknown factor, which is running every minut or so, keeps the disk active. I am running ext2 in order to avoid journal writes.

I have tested with iotop, but iotop doesn't reveal what process is activating the drive, it only revales that some disk activity occurs, but not whats causing it.

All processes including rsyslog has been deactivated.

# sdparm -C sync /dev/sda
# sdparm -C stop /dev/sda

This is an output from "iotop -o" while a disk write occurs:

Total DISK READ : 0.00 B/s | Total DISK WRITE : 0.00 B/s
Actual DISK READ: 0.00 B/s | Actual DISK WRITE: 3.85 B/s

No process is revealed.

How can I figure out whats keeping this disc active?