does your script run under the same user as when you called
echo 100 > /sys/devices/platform/wdc-fan/speed ?
if yes, I don't know, but if no, then please check the rights of this user or let the script run under the same user.
I have no experience with the temperature_monitor script. But are you sure that the script really tries to change the value in /sys/devices/platform/wdc-fan/speed ?
Perhaps you should write some messages to an own logfile every time the script tries to change the value. Only to be sure that you do not look for errors that don't exist or are somewhere else.
Your question about hdparm. I assume you have a firmware 2.00.xx. I'm right?
To check your current version call
hdparm -V (note the capital V)
I'm not sure if an old version of hdparm could be this problem. The best/easiest way to install is using optware. Please look in wiki for optware installation. And keep the hints at http://kyyhkynen.net/stuff/mybook/spindown.php in mind (there must be a path to hdparm in the script).