105 / hour on battery :-(
AFAIK, some disks need very high values to limit load cycling.
Try again with 254 or even 255, if it's needed.
=> 1st question : LED blinking doesn't mean start/stop drive, OK, but is it dangerous for disk ?
I have tried 'iotop' : guilties could be firefox or kjournald2 (often accessing disk when I don't do anything on computer)
What filesystem are you using?
Anyway, there are so many reasons for the disk to be accessed, it's hard to figure out what's going on.
Please attach a new dmesg, taken after some time of work. Let's see if anything strange happens to the disk during this period.
=> something else to verify ?
Well, nothing else comes to my mind right now.
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