Needed parts and tools:
Level of difficulty: easy
Many forum posts and articles describe that the GuruPlug gets very hot. Also mine gets quite hot (only measured by hand, but hot enough to be alarmed). This is why I needed a solution to at least cool it a little. Since I did not want to open the case and lose warranty, I took a 12cm 12V fan of my old PC and connected it to the power pins of an USB-plug. Now I have a quiet fan and it cools my GuruPlug pretty good (at least on the outside). The fan needs about 80mA which should be not problem for the USB power. It seems like that the USB-interface on the GuruPlug does not has enough power to run an external harddisk powered over USB, but running the fan works and should not cause any problem.
Usually, the fans of a PC have three or four wires. The fan I use is a chassis fan with three wires. A black one, a red one and a yellow one. The black one and the red one are the power wires which I attached to the power pins of the USB-plug.
The following picture shows the pin description of the USB-plug. Only the - and + pins are needed.
If you are using heat shrink tubes, don't forget to put them on the wires first. Then solder the wires of your fan to the power pins on the back of the USB-plug. The red wire of the fan to the + of the plug, and the black wire of the fan to the - of the plug. If you switch the + and the -, nothing will happen except that your fan might run backwards (or not run at all).
Advice: Before you connect the fan to your GuruPlug, connect the fan to a power supply or anything else that you can use to measure current. USB has a maximum current of 500mA, but the GuruPlug probably should not be loaded with that much. A current around 100mA should be alright.