ARCHIVED: In Mac OS X, what does the startup icon mean?

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When you power up a Mac, an icon will appear indicating whether the operation was successful. Icons include:

Gray apple or Happy Mac: No problems detected; the system will continue to boot normally.
Flashing question mark: The computer is waiting for you to insert a startup disk, or doesn't recognize your internal hard drive as a valid startup disk.
Prohibitory sign or broken folder: Important files in the operating system have been damaged or removed.
Flashing globe: You are trying to start from a netboot server that doesn't exist or is offline.