Do you use a skin that closely monitors CPU-temp/fanspeed, etc.? This might interfere.
Try the default PLI-skin and verify whether it is solved now.
Another possibility: Did you park the picon-set on the HDD or on sd-card (internal cache). Keep 'em on sd-card.
I have installed picons on usb as i have mentioned in my earlier post. I do use Ai HD skin which gives the DB temperature on the navigation info bar. But I can't get the HDD temperature in this skin. Does it awake up HDD to get temperature from its sensor ? But as you said i will test it with the default PLi skin. Also i am using the same skin in VIX with HDD going to standby after 5 min.