Back in August, I did an OS update on my iMac, and the machine wouldn't reboot. I was able to reinstall Leopard by booting from a disk, and all was well for a few more weeks. The machine seemed to run normal, with the exception of 5-minute-plus boot up times. But within a few weeks it stopped booting. I installed Leopard on an external HDD and figured I'd leave the internal alone until Snow Leopard came out. A few weeks later when SL was released, I booted from the install disk, found the internal HDD, and went through the installation process. Everything went fine but the machine would not boot. I installed SL on my external HDD running Leopard, and everything was fine. I've since been running my computer off the external. The internal HDD no longer shows up on the desktop, in Disk Utility, etc. I'm fearing it may have died. More worrisome, though, is my machine doesn't always boot up, whether from my external HDD (set as the startup disk) or from CD. By "doesn't always boot up," I mean most of the time when I restart, it just sits on a gray screen, nothing happens (even when holding "Option" or C.) Sometimes it will boot, but takes 20 or 30 minutes to do so. I'm considering replacing the internal HDD, but am curious, does that seem like the issue, or might the failure to boot from externals and CDs indicate a more serious problem?