Had my iMac G5 for a few weeks when it showed the question mark folder icon after starting up. Went through 1/2 hour of troubleshooting with Apple phone support before they said I should take it to an Apple Store. When they plugged it in at the store, it started up fine. They ran diagnostics, opened it up, and couldn't find any problems. The only thing the tech person mentioned was maybe it had something to do with the fact that I was using File Vault but didn't know for sure. I just took it home and everything's working well. I'm wondering if anyone else has had a similar experience.