Yesterday I came home to find the screen of my mac frozen on the default screensaver, with a message in a gray box saying I had to restart my computer; The message was clearly from the OS (in several languages) but I've never seen it before. I restarted and it appeared again quite quickly. Rebooted, it happened again. This seems quite worrying. Does it mean my HD is dying? I wasn't able to run any diagnostic tests on it, the computer just freezes.