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It happened to me twice recently. I was just using my iMac normally, like browsing the web, and it suddenly freezes. I can move the mouse, but I can't click on anything. Keyboard not functioning at all. I couldn't figure out what to do except for pressing the power button and reboot. What else can I do? Is there a way to diagnose and find out what is causing it?

5

A strategy to try when the freeze is actually happening is holding ⌘ Cmd and ⌥ Opt, then hitting ⎋ Escape.

If your keyboard input is still working, this should bring up the force-quit dialog (which can be navigated using the arrow keys and Enter). This may allow you to close problematic applications without a hard shutdown using the ⌽ Power button.

1

Usual troubleshooting steps for similar cases are:

  1. Clean system and user cache (slightly dangerous procedure if you don't know what you're doing) there is a program called CleanGenius that might help, but it's been severely limited due to the new sandboxing policies introduced with OS 10.7
  2. repair Disk Permissions (easy, you can do it with Apple's Disk Utility, which is installed by default in the Utilities folder)
  3. Reset PRAM (easy, you need to start up your computer keeping four keys pressed: command, option, P, R and release them AFTER you heard the startup chime TWICE)

Those steps should solve the problem.

Differential diagnosis would be checking the RAM for a defective bank and checking internet plugins (but in that case, I suppose not all the system would lock up, just the browser) and processes running in the background. But for this kind of diagnosis, I would suggest entrusting your Mac to a specialist.

0

Try cleaning the dust bunnies out of the video card. After months of making me crazy I think I tracked it down to dust blocking the heat sink channels on my video card. I was blaming my freezed on Flash, but I realize that was working my video card just hard enough it got hot and quit. Haven't had a freeze in days.

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