Finding out what is spinning up harddrive

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http://unix.stackexchange.com – I am the owner of a NAS, running some Linux distribution. It comes with a web administration frontend, where i can manage several services, user rights and also when it should go to sleep. My problem is, for some reason, when the NAS has gone to sleep, the harddrive turns on again after a couple of minutes. Then it will spin for some time, then sleep again. This keeps going on infinitely. How can i try to determine the cause for this? I am very new to Linux, but i managed to get root access, and now have a SSH connection. Thanks (HowTos)