I'm pretty sure it means the system can not find the boot drive, but... My iMac stays on 100% of the time, but a few months ago I turned it off for about 18 hours during a storm. When I turned it back on, I got the flashing question mark. I turned it off & back on about 8-10 times, when it finally booted up just fine. It never does it at a reboot, only if off a while. For several months, it did this any time it was off a while. About 6 weeks ago, it stopped doing it! Now, I can't MAKE it happen! I should feel lucky, but I'm also nervous about an impending failure. When SL came out, I repartitioned the drive, first going to MBR, then back to GUID, just to re-write everything. That neither helped (the issue was already gone), nor hurt. It still won't ever do it. Fluke, or should I be planning for a drive failure?