What can cause the hard drives to start "grinding" (writing) every night?


As the subject says - every night, shortly before midnight, I hear the hard drive(s) in my EX2 Ultra start writing. I don’t write or read anything and neither is anyone of the users.
I disconnected the Ethernet cable - it stopped.
I reconnected it - it started again (after a minute or so).
What can it be and how do I find out?

Also, is there a way to see what changes were made to the drive, what was written to the drive (like last files/folders)?