I recently installed CoreTemp and SpeedFan. When my system is idle, my CPU is stable at about 34C to 35C. I noticed when I turn on the external HDD, the CPU temp jumps 10C and remains there as long as external drive is on. During this time, CoreTemp reports the CPU is operating at 0% to 1% load. Same behavior if I plug in SanDisk.

So if the CPU is idle, what could be causing core temperature spike? Do others see this?

A8V Deluxe, Athlon 64 X2 4200+

I believe there was a bug in CoreTemp 0.99 that related to this.
Try CoreTemp 0.99.1

Thanks for your reply. I'm already running 99.1. The issue I described is verified by SpeedFan so there's little doubt it's an actual temperature spike.

Since posting my original message, I discovered my problem is due to a bug in a microsoft USB driver installed by XP SP2. When a USB device is connected, the driver leaves a process running that prevents the CPU from entering a lower power state. I was able to fix by adding a registry entry, as described in the following link.

http://it.slashdot.org/comments.pl?sid=175466&cid=14588517

Apparently, this issue and the fix was original documented three years ago as it had severe impact on battery life in portable computers.

HA.
Well I never, ever use the speed stepping or powersaving CPU settings on my desktop, though I do use the monitor and HD sleep functions, so this never occured to me.
Under 100% load I run around 39-43C anyway so it didn't jump out at me.

Good catch