hard disk spin down

[pol <linux_milano@yahoo.it>]

Most of the time spent on my laptop is reading. Often the laptop is not
powered, so that i can hear the inner disk spinning down and restarting
every dozen seconds, because many processes are running, while i am
reading.  
My question is: is there a way to force processes not to read and write so
often? 
Alternatively, is it safe disabling the disk spin down? 

My understanding is that time to spin down is set
in  /etc/default/acpi-support, where SPINDOWN_TIME=12 
and controlled by the kde-guidance-powermanager package. Is that correct?
Why so a short time (12s) is set as the default?


Thank you 
--
Pol