I just started having this problem on my Mac Pro running Snow Leopard OS 10.6.4 - when I boot my computer, the gray multi-language screen comes up which says "You need to restart your computer. Hold down the power button for a few seconds..." Apparently this is called a kernel panic. There is another thread about this but since it was 5 years old and using a different OS I decided to create a new one. The other thread is here: http://forums.macrumors.com/archive/index.php/t-105221.html On that thread someone says a kernel panic is almost always a hardware problem. It's recommended to run a Hardware test. Well, the regular hardware test showed no problems. The extended hardware testing has been going on for over 2 1/2 hours and i'm not even sure if the blue status bar is moving anymore. The estimated time to complete was "30 minutes or longer, depending on the amount of memory you have installed." I have about 2TB of internal HDD space and 12GB of RAM so I could see it taking longer than 30 minutes, but 2 1/2 hours?!? I'm not sure if it's frozen so I don't want to click Stop Testing unless I'm sure it's not still working. So I'm hoping someone has experience with the Hardware Test and can tell me either "It's normal for a Hardware Test to go on over 2 1/2 hours," or "Hardware Tests should never last that long. It must be frozen." I should also note that underneath the status bar it says "Testing Memory." So maybe if it is frozen, it means something is wrong with the memory?