2. Next, try starting up into the Extensions Manager (hold the Space Bar down and start up the computer).
When the Extension Manager window appears, select the "Mac OS 8.x All" set of extensions and allow the system to continue starting up. If that fixes the problem, then you know the problem is an extension conflict.
3. If the error still occurs, the next thing you should try is starting up from a different source, such as the iMac install CD or any other bootable system software volume.
If the computer starts up properly from the CD (or other startup volume), then you can be confident that the computer does not have a hardware problem.
4. The next thing you should try is updating the hard disk driver using Drive Setup.
After updating the hard disk driver on the suspect hard disk, try restarting. If the computer still exhibits the problem continue with the steps below.
For more specifics on the hard disk driver, please see Knowledge Base article 24585: "Hard Disk Driver: Identifying It And Ensuring Compatibility"
5. Consider resetting Parameter RAM (PRAM); sometimes the issue can be resolved by performing this procedure.
For instruction on resetting Parameter RAM, please see Knowledge Base article 2238: "Macintosh: How To Reset PRAM And NVRAM"
6. If the computer started up and ran properly from a different volume (as described in step 3 above), and the computer still exhibits the problem then the last step is to perform a clean install of the system software.