A Happy Mac is the normal bootup icon of an Apple Macintosh running the classic Mac OS (not Mac OS X however). When you boot one up, it makes a beep noise, and the screen turns gray. If everything goes okay during the diagnostic tests, a happy Mac icon will appear, followed by the splash screen of whatever version of the Mac OS you are running. If the diagnostics fail, a Sad Mac icon will appear instead, signaling that there has been a problem in the bootup sequence.
On early Macs that had no internal hard drive, the computer would boot up to a point where it would need to load the operating system from floppy disk. If there was no disk present, it would display a blinking disk icon instead. This is normal. The blinking disk is still sometimes seen on hard drive-based systems where the disk is unbootable for some reason (such as being fully erased), though there is no other fault that prevents the system from starting up. The blinking disk icon will appear after the happy Mac icon has been shown, though that may only be shown briefly.
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