This is driving me insane; the problem itself is fixed, but I do not understand AT ALL what the problem was. So here are the symptoms: My client came to me with a Macbook that wouldn't boot. Most of the time it hung on the gray screen, occasionally it would get to the "blinking folder" stage. It wouldn't boot into target disc (firewire) mode, wouldn't boot into Single User or Verbose modes, wouldn't boot from a dvd or cd-rom, it was just dead. Tried every trick in the book (playing around with the RAM configuration, resetting PMU [or CMS or whatever it is in the Intel models], trying to boot into the open firmware, etc) and nothing worked. At all. This machine was dead. The only thing it would boot was the hardware test disc, and it passed every test, then failed to boot into the OSX install DVD that Hardware Test is on. So we tried replacing the hard drive with an old Tiger hard drive. And boom, everything worked. Solution: bad hard drive. Here's what is driving me insane--some of those solutions (booting from various DVDs, booting into Open Firmware, Target Disc mode, etc.) should, in theory, work regardless of whether or not the hard drive is dead. Anyone have any idea why this would happen?