HDD spins down every few seconds

[event]

Hi,

I'm using Knoppix 7 as hdd install right now on my notebook.
Everytime its running on battery the harddrive is spinning down every few seconds. Always needs some time to spin up again, and I'm sure its not helping its lifetime.
Also probably not saving any battery, rather using more.

I already tried hdparm -S255 /dev/sda , but no help.
Any ideas?