My Mac repeatedly beeps three times on startup. What does this mean?
My Mac repeatedly beeps three times on startup. What does this mean?
When I try to boot my Mac I get nothing but strange beeps. They beep as follows:
BEEP BEEP BEEP [3 second pause] BEEP BEEP BEEP [3 second pause] and it continues until I power off.
So far, I have:
Is my Mac possessed? Assuming not, what is it trying to tell me?
Background
For various reasons I want to re-install the original Mac OS X. To do this I am trying to boot from an original Snow Leopard disc in the SuperDrive. I have no reason to suspect a problem with either the disc (no scratches or other signs of damage and it boots a Mid 2009 MacBook Pro just fine) or the optical drive (it loads other discs fine).
The Mac boots fine otherwise (both normally and in Safe Mode), but just doesn’t want to boot from this disc and I have no idea why, but I’m pretty sure the beeps are the key! :)
Specifications:
Update
Upon checking after Monomeeth's answer I can add that the original Snow Leopard disc I am trying to boot from is indeed a retail version containing 10.6.3. 
@V2Blast Thank you for the explanation. :)
– user301113
Sep 6 '18 at 12:46
1 Answer
1
No, your Mac is not possessed. And Yes, it’s trying to tell you something!
A looping three beeps in between three seconds during startup is your iMac’s way of telling you that the operating system you’re trying to boot into is incompatible with your Mac hardware.
In other words, the Snow Leopard disc you’re trying to boot from contains a version of Snow Leopard that is earlier than version 10.6.6 (the earliest your iMac can support).
My guess is that you’re not using the grey disc that shipped with your iMac. It’s either one that belongs to another Mac or it’s a retail version that’s earlier than 10.6.6.
The only ways to get around this is to use another disc or do the following:
WARNING: You should always ensure you have a backup of your data, especially when performing operations such as this!
I'd be hesitant to recommend this—if it doesn't boot from the CD, is it really likely to boot from the disk?
– Kevin
Sep 5 '18 at 16:18
@Kevin: A similar process for installing Windows 10 on older iMacs works just fine (where Bootcamp is limited to Windows 8), so it might work okay.
– user286843
Sep 5 '18 at 17:52
@Kevin Thanks for the comment. The beeping pattern described by the OP definitely means the OS isn’t compatible. Since they’re trying to install Snow Leopard, we know it must be version 10.6.0 to 10.6.5 they’re trying to boot from as their iMac does support 10.6.6 to 10.6.8. So, assuming there’s no other unknown issues, then following my steps will work as long as they remember to do Step 5 (as that’s what will update the installed version to a compatible one). Hopefully the OP will report back either way. If you have any other questions please feel free to ask. :)
– Monomeeth♦
Sep 5 '18 at 22:30
@user301113 Yep, it certainly fits! And thank you for coming back to share your results, it's sure to help others down the track! :)
– Monomeeth♦
Sep 6 '18 at 4:46
@Monomeeth You're a legend! I followed your steps and my Mac is now purring nicely on Snow Leopard. It was actually very easy to do. :)
– user301113
Sep 6 '18 at 12:45
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– V2Blast
Sep 6 '18 at 7:51